prune old string dtype class

[d-v-b]

When we supported NumPy versions < 2, we had to implement the variable-length string dtype on top of the numpy "object" dtype. For NumPy versions > 2, we use the StringDType.

Since we no longer support numpy < 2, we can remove the variable-length dtype class that used the "object" dtype. that's this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.34%. Comparing base (e815b51) to head (e4c3fda).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3973      +/-   ##
==========================================
+ Coverage   93.29%   93.34%   +0.05%     
==========================================
  Files          87       87              
  Lines       11752    11739      -13     
==========================================
- Hits        10964    10958       -6     
+ Misses        788      781       -7     

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/string.py	97.69% <100.00%> (+5.38%)	⬆️

... and 1 file with indirect coverage changes

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