Moved freeze_panes validation to io/excel.py (#15160)
 - follow up to #15160 
 - [x] tests added / passed - Existing test suite passed
 - [x] passes ``git diff upstream/master | flake8 --diff``


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15592?src=pr&el=h1) Report
> Merging [#15592](https://codecov.io/gh/pandas-dev/pandas/pull/15592?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0bf45320440e0d477f6b31ac8825f34e8212f152?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `88.88%`.


```diff
@@            Coverage Diff             @@
##           master   #15592      +/-   ##
==========================================
- Coverage   91.06%   91.03%   -0.03%     
==========================================
  Files         137      137              
  Lines       49307    49307              
==========================================
- Hits        44900    44889      -11     
- Misses       4407     4418      +11
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15592?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/0bf45320440e0d477f6b31ac8825f34e8212f152...81cb86f00b15df15e9b5c389e838c5cc4d6761e0?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <ø> (-0.06%)` | :x: |
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/compare/0bf45320440e0d477f6b31ac8825f34e8212f152...81cb86f00b15df15e9b5c389e838c5cc4d6761e0?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `79.67% <88.88%> (+0.03%)` | :white_check_mark: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/0bf45320440e0d477f6b31ac8825f34e8212f152...81cb86f00b15df15e9b5c389e838c5cc4d6761e0?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/0bf45320440e0d477f6b31ac8825f34e8212f152...81cb86f00b15df15e9b5c389e838c5cc4d6761e0?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15592?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/15592?src=pr&el=footer). Last update [0bf4532...81cb86f](https://codecov.io/gh/pandas-dev/pandas/compare/0bf45320440e0d477f6b31ac8825f34e8212f152...81cb86f00b15df15e9b5c389e838c5cc4d6761e0?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). thanks @jeffcarey 