remove usage of the _deprecate_positional_args decorator, and tests for its behavior

[d-v-b]

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.61%. Comparing base (7154f98) to head (77a9b16).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3225      +/-   ##
==========================================
- Coverage   94.76%   94.61%   -0.15%     
==========================================
  Files          78       78              
  Lines        8672     8643      -29     
==========================================
- Hits         8218     8178      -40     
- Misses        454      465      +11     

Files with missing lines	Coverage Δ
src/zarr/api/synchronous.py	92.85% <ø> (-1.81%)	⬇️
src/zarr/core/array.py	98.37% <ø> (-0.15%)	⬇️
src/zarr/core/group.py	94.81% <ø> (-0.07%)	⬇️

... and 3 files with indirect coverage changes

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[d-v-b]

there's a reduction in coverage, but the previously covered lines of code were not actually being tested for correctness -- they were just being run to check the behavior of the positional args decorator. So I think we can disregard the coverage reduction here.

[d-v-b]

this failure is due to recent changes to our hypothesis tests, presumably revealing a real bug, but likely one unrelated to this PR (cc @dcherian)