python: remove `CPATH` from `setup_run_environment` (#34727)

Currently, the `python` package tries to set `CPATH` in `setup_run_environment()`.

We no longer set `CPATH` and other destructive environment variables (like
`LD_LIBRARY_PATH`) in modules, so we shouldn't do something special for Python.

Also, the way `python` sets `CPATH` causes issues. Because it does a header search to
find directories containing headers, if you bootstrap `mypy` or other style tools on a
fresh Ubuntu image with *no* python devel headers installed, you'll get an error like
this when trying to load the thing you just installed:

```console
[root@980de539843d /]# spack -b load py-mypy
==> Error: Unable to locate python headers in any of these locations:
/usr/include/python3.6m
/usr/include/3.6
/usr/Headers
```

The headers and includes aren't needed to get `mypy` in the path or for `mypy` to work,
so we're failing unnecessarily here.

- [x] remove `setup_run_environment()` from `python/package.py`

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