Simplify scoped loop fixture discovery

[seifertm]

Historically, pytest-asyncio maintained one event loop fixture per pytest collector. This has changed in #1107 and pytest-asyncio only maintains one event loop per scope.

This PR removes obsolete management code related to having one loop per collector. In the process, this removes the restriction that a test marked with asyncio(loop_scope="class") or asyncio(loop_scope="package") has to be surrounded by a class or test. The new behavior is in line with pytest's scope argument to fixtures.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.63%. Comparing base (b0f9cc0) to head (88e8454).
⚠️ Report is 231 commits behind head on main.

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1112      +/-   ##
==========================================
- Coverage   90.14%   89.63%   -0.52%     
==========================================
  Files           2        2              
  Lines         467      434      -33     
  Branches       60       53       -7     
==========================================
- Hits          421      389      -32     
  Misses         30       30              
+ Partials       16       15       -1     

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