Remove src/numpy.pxd
The existence of the src/numpy.pxd file causes ambiguity as to where cimports are coming from.  src/numpy.pxd?  cython's numpy/__init__.pxd?  What about the np.get_include() uses in setup.py?

This PR gets rid of the unnecessary file by picking out the one definition that is actually needed.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19418?src=pr&el=h1) Report
> Merging [#19418](https://codecov.io/gh/pandas-dev/pandas/pull/19418?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5f6c80b5e3641ab96f747bd9967e4d58b5852814?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19418/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19418?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19418   +/-   ##
=======================================
  Coverage   91.65%   91.65%           
=======================================
  Files         150      150           
  Lines       48724    48724           
=======================================
  Hits        44658    44658           
  Misses       4066     4066
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.02% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19418?src=pr&el=footer). Last update [5f6c80b...90e7516](https://codecov.io/gh/pandas-dev/pandas/pull/19418?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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