ENH: Remove StatelessActor

[codingl2k1]

What do these changes do?

The StatelessActor is ambiguity, remove it and use a class attribute __xoscar_lock_type__ instead.

Related issue number

Fixes #67

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7f3de4) 89.02% compared to head (7e68a37) 89.02%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   89.02%   89.02%           
=======================================
  Files          48       48           
  Lines        4010     4010           
  Branches      764      764           
=======================================
  Hits         3570     3570           
  Misses        355      355           
  Partials       85       85           

Flag	Coverage Δ
unittests	88.85% <ø> (ø)

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