Remove inline declarations in pxd files
At the moment (I think since .26, not sure) they don't do anything but cause lots of warnings during cythonizing.  Eventually they will raise errors.

https://github.com/cython/cython/issues/1706#issuecomment-302347613

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=h1) Report
> Merging [#17277](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/24b6349c013fb9e59ea7fa4b1d40088026c32d25?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17277/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17277      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.02%     
==========================================
  Files         162      162              
  Lines       49547    49547              
==========================================
- Hits        45091    45083       -8     
- Misses       4456     4464       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17277?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/17277?src=pr&el=footer). Last update [24b6349...1fc1b7d](https://codecov.io/gh/pandas-dev/pandas/pull/17277?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/17277?src=pr&el=h1) Report
> Merging [#17277](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/24b6349c013fb9e59ea7fa4b1d40088026c32d25?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17277/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17277      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.02%     
==========================================
  Files         162      162              
  Lines       49547    49547              
==========================================
- Hits        45091    45083       -8     
- Misses       4456     4464       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17277?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/17277?src=pr&el=footer). Last update [24b6349...1fc1b7d](https://codecov.io/gh/pandas-dev/pandas/pull/17277?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 this would need a performance check. older versions of cython may respect this directive. The comment above only seems to apply to *cython* defintiion, NOT c definitions.  > The comment above only seems to apply to cython defintiion, NOT c definitions.

OK, then this is moving outside my competence zone.  We can wait until it starts raising errors and fix it then. ok, as I said its new in cython 0.26 if you want to specifically address some things great. but will need performance checking.