Remove the sample project MANIFEST.in reference

[DiddiLeija]

If pypa/sampleproject#155 gets merged, the link may get broken, and also may cause a confussion. So, this PR removes the only reference to that file, and also explained why.

[DiddiLeija]

Ok, the tests failed:

/home/runner/work/packaging.python.org/packaging.python.org/source/guides/installing-scientific-packages.rst:16: WARNING: unknown document: scipy:stackspec
/home/runner/work/packaging.python.org/packaging.python.org/source/guides/installing-scientific-packages.rst:92: WARNING: undefined label: scipy:mac
/home/runner/work/packaging.python.org/packaging.python.org/source/guides/installing-scientific-packages.rst:101: WARNING: unknown document: scipy:install

However, this seems to be external to this PR, right?

[DiddiLeija]

However, this seems to be external to this PR, right?

Oh, I see that #1006 wants to fix this issue.

[DiddiLeija]

Excluding the above issue, do I have to do something else?

[webknjaz]

@DiddiLeija wait for the reviews, I guess.

[DiddiLeija]

Thanks @webknjaz!