remove 'tomli' dependency

[jameslamb]

Proposes removing this project's build-time dependency on tomli.

It appears to no longer be necessary.

git grep tomli

Notes for Reviewers

I originally noticed something similar in ucx-py (https://github.com/rapidsai/ucx-py/pull/1042), then went searching for similar cases across RAPIDS.

I'm not sure why this project has a dependency on tomli, but I suspect it was related to the use of versioneer in this project's history. Reference: https://github.com/python-versioneer/python-versioneer/issues/338#issuecomment-1381170813

This project doesn't use versioneer any more (#1204). I strongly suspect that the dependency on tomli can be removed.

[jakirkham]

/merge

[jakirkham]

Thanks all! 🙏