Drop `requirements.txt` from package lock files

[nicholasjng]

It serves no purpose over requirements-dev.txt, which we advise installing for contributors, and is unused in package distribution.

Also clean up a stale pre-commit comment, and remove the requirements.txt compile hook.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (39f249a) 88.04% compared to head (e922330) 88.04%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #194 +/- ## ======================================= Coverage 88.04% 88.04% ======================================= Files 7 7 Lines 485 485 Branches 70 70 ======================================= Hits 427 427 Misses 31 31 Partials 27 27 ```

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[nicholasjng]

Rebased, since the requirements.txt deletion ended up in the first (dev and docs update) commit.