chore: remove MANIFEST.in

[kiyoon]

Sorry, I forgot to remove this file which is not used anymore.

FYI, to ensure the build, I ran the build consistency test I wrote recently.

$ version-pioneer build-consistency-test
12:57:02 INFO     version_pioneer - Running with version-pioneer 0.0.5                     __init__.py:191
uv 0.5.11 (Homebrew 2024-12-20)
         INFO     version_pioneer.api - Testing build consistency...                            api.py:255
         INFO     version_pioneer.api - Changing cwd to /Users/kiyoon/project/pdb2pqr           api.py:261
         INFO     version_pioneer.api - Building the project with `uv build`                    api.py:266
12:57:12 INFO     version_pioneer.api - Building the project with `uv build --sdist` and `uv    api.py:275
                  build --wheel`
12:57:21 INFO     version_pioneer.api - Built version: 3.6.2                                    api.py:310
         SUCCESS  version_pioneer.api - ✅ 2 wheel builds are consistent.                  __init__.py:161
         SUCCESS  version_pioneer.api - ✅ 2 sdist builds are consistent.                  __init__.py:161
         INFO     version_pioneer.api - Building the project with `uv build --sdist` using the  api.py:349
                  built sdist (chaining test).
         INFO     version_pioneer.api - Changing cwd to the built sdist directory:              api.py:353
                  /var/folders/r5/9cpfjfjx3b73b6stl7_w712h0000gn/T/tmpuq0un20e/dist/pdb2pqr-3.6
                  .2
12:57:31 SUCCESS  version_pioneer.api - ✅ Chained sdist builds are consistent.            __init__.py:161
         INFO     version_pioneer.api - Build wheel using the sdist.                            api.py:404
         SUCCESS  version_pioneer.api - ✅ sdist -> wheel chained build is consistent with __init__.py:161
                  the non-chained build.
         SUCCESS  version_pioneer.api - 💓 All tests passed! 3 sdist builds and 3 wheel    __init__.py:161
                  builds are consistent.
         INFO     version_pioneer.api - Deleting temporary directory                            api.py:443
                  /var/folders/r5/9cpfjfjx3b73b6stl7_w712h0000gn/T/tmpuq0un20e

See https://github.com/kiyoon/version-pioneer?tab=readme-ov-file#version-pioneer-build-consistency-test-test-build-consistency if you're interested