Simplify vcs version handling

[grst]

One downside of the VCS-based versioning is that when the project
is installed in editable mode, the version encoded in the package
metadata will remain the one from installation time and get stale.

There was some additional code in MuData that attempted to address this,
but I believe it does more harm than good:

• while it would have updated __version__ (which is a convention, but
  not a standard), the standard way to retrieve the version via
  importlib.metadata would still have returned the version number
• It goes hunting for the version number at import time and
  things can go wrong while doing so. In fact, building the conda
  recipe for scirpy fails on CI exactly because some dependency for
  getting the version at runtime is missing
  (https://github.com/scverse/scirpy/actions/runs/21799752388/job/62893203310?pr=672)
  or it might fail because one has a shallow clone without git history
  etc.

With this PR, I propose to get rid of all the additional version
handling and be more aligned with the cookiecutter template. Risking
stale metadata in editable mode is a trade-off that we conciously took
in the template, because the consequences are low-impact and only affect
developers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.50%. Comparing base (b047610) to head (129aa29).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
- Coverage   68.57%   68.50%   -0.08%     
==========================================
  Files          12       11       -1     
  Lines        1906     1889      -17     
==========================================
- Hits         1307     1294      -13     
+ Misses        599      595       -4     

Files with missing lines	Coverage Δ
src/mudata/__init__.py	100.00% <100.00%> (ø)

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