 CLN: Panel reference from documentation
xref #25632

I didn't remove Panel reference from the whatsnew docs. Figured they should still be kept there for users who install older versions.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=h1) Report
> Merging [#25649](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f88613929d6c83952cb5a6bf70678a78be480baa?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25649/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25649      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         173      173              
  Lines       52968    52968              
==========================================
- Hits        48340    48339       -1     
- Misses       4628     4629       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25649/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=footer). Last update [f886139...92ec876](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=h1) Report
> Merging [#25649](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/46639512c06300a9844ea27f90167d5648c9b93a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25649/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25649      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         173      173              
  Lines       52982    52982              
==========================================
- Hits        48356    48355       -1     
- Misses       4626     4627       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.76% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25649/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25649/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=footer). Last update [4663951...8829954](https://codecov.io/gh/pandas-dev/pandas/pull/25649?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. All green.  Thoughts on keeping a small piece of panel.rst around saying "Panel was removed in pandas 0.25.0. See <previous version> for documentation?". That would at least avoid breaking links for people pointing to `/stable`. The doc build is broken here. It doesn't look like to be related to the changes in this PR (a tornado / jupyter compatibility), although I don't see it yet on other PRs on travis. But anyway, I would wait with merging until that is fixed (opening a PR shortly) Hmm, just restarting the build also seems to have fixed it.

Some additional warnings about no longer existing labels from the logs that should be fixed:

```
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.20.0.rst:1372: WARNING: undefined label: dsintro.deprecate_panel (if the link has no caption the label must precede a section header)
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.23.0.rst:660: WARNING: undefined label: dsintro.deprecate_panel (if the link has no caption the label must precede a section header)
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.6.0.rst:42: WARNING: undefined label: basics.panel (if the link has no caption the label must precede a section header)
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.0.rst:63: WARNING: undefined label: dsintro.to_panel (if the link has no caption the label must precede a section header)
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.0.rst:75: WARNING: undefined label: dsintro.panel_item_selection (if the link has no caption the label must precede a section header)[39;49;00m
/home/travis/build/pandas-dev/pandas/doc/source/whatsnew/v0.7.0.rst:99: WARNING: undefined label: dsintro.panel_item_selection (if the link has no caption the label must precede a section header)
``` Kept `panel.rst` and left a note regarding the removal and should have fixed all the errant doc warnings.  thanks @mroeschke 

pls review built docs for clarity of this patch.