Remove unused imports

[archang19]

Summary

At first, I ran CLANGTIDY against the whole repo and accepted all of the recommendations, but that created too many errors to handle.

I then filtered out only unused import errors. The proposed changes were mostly correct, but I had to do some manual fixes at the end to get CI to pass.

NOTE: At least in its current form, I don't think we want to merge this PR because it looks like CLANGTIDY has some false positives that do not result in CI checks failing. In particular, if we include headers X and Y, and Y also includes X, then even if we use a dependency specified in header X, it looks like CLANGTIDY still tries to get rid of the include. In this case, we do not want to rely on a transitive inclusion.

Test Plan

CI