DEPR: Deprecate Series.valid
- [x] xref #18262
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This PR deprecates the ``valid`` method on ``pd.Series``. This method duplicates ``dropna`` and should be deprecated according to #18262.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18800?src=pr&el=h1) Report
> Merging [#18800](https://codecov.io/gh/pandas-dev/pandas/pull/18800?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0303d0d6cd983758fbe5c4618ceabf1b880a95f4?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18800/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18800?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18800      +/-   ##
==========================================
- Coverage   91.63%   91.62%   -0.02%     
==========================================
  Files         154      154              
  Lines       51422    51430       +8     
==========================================
- Hits        47123    47122       -1     
- Misses       4299     4308       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.49% <100%> (ø)` | :arrow_up: |
| #single | `40.83% <25%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18800?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.82% <100%> (ø)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.33% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.9% <0%> (ø)` | :arrow_up: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/18800/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `94.54% <0%> (+0.66%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18800?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/18800?src=pr&el=footer). Last update [0303d0d...8c534ca](https://codecov.io/gh/pandas-dev/pandas/pull/18800?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @topper-123! Thanks for updating the PR.























Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 15, 2017 at 23:40 Hours UTC lgtm. ping on green. I'm using ``FutureWarning`` as the warning type. Should this not be ``DeprecationWarning`` instead? I could update my previous deprecation commits also, while I'm at it. no we always use FutureWarning 
DeprecationWarning is not shown by default All green