Remove newstyle variable given old-style class support should be removed

[akamat10]

Type of Changes

	Type
✓	✨ New feature
✓	🔨 Refactoring

Description

Remove newstyle from ClassDef and code associated with old-code style. Given newstyle will no longer be available, code externally accessing this will break and will need to stop using this including pylint. I haven't cleaned up postinit and visit_classDef to remove the newstyle parameter. Open to whether the should be removed too.

Closes #2560

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.05%. Comparing base (e442776) to head (b8dd663).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2561      +/-   ##
==========================================
+ Coverage   92.99%   93.05%   +0.06%     
==========================================
  Files          93       93              
  Lines       11086    11046      -40     
==========================================
- Hits        10309    10279      -30     
+ Misses        777      767      -10     

Flag	Coverage Δ
linux	92.93% <100.00%> (+0.06%)	⬆️
pypy	93.05% <100.00%> (+0.06%)	⬆️
windows	93.03% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
astroid/helpers.py	95.36% <100.00%> (-0.10%)	⬇️
astroid/interpreter/objectmodel.py	93.81% <ø> (+0.53%)	⬆️
astroid/nodes/scoped_nodes/scoped_nodes.py	93.11% <100.00%> (+0.30%)	⬆️
astroid/objects.py	94.26% <ø> (+0.55%)	⬆️