Remove unused constants

[cdce8p]

Description

Remove PY38_PLUS and previously deprecated BUILTINS constants.
Also remove Load, Store, and Del constants as those have already been aliases for Context enum.

[codecov]

Codecov Report

Merging #2141 (952868b) into main (7fa8481) will increase coverage by 0.25%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2141      +/-   ##
==========================================
+ Coverage   92.23%   92.48%   +0.25%     
==========================================
  Files          94       94              
  Lines       10889    10837      -52     
==========================================
- Hits        10043    10023      -20     
+ Misses        846      814      -32     

Flag	Coverage Δ
linux	92.24% <100.00%> (+0.25%)	⬆️
pypy	87.51% <100.00%> (+0.23%)	⬆️
windows	92.08% <100.00%> (+0.25%)	⬆️

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Impacted Files	Coverage Δ
astroid/const.py	100.00% <ø> (ø)
astroid/__init__.py	100.00% <100.00%> (ø)
astroid/bases.py	88.04% <100.00%> (-0.07%)	⬇️
astroid/brain/brain_ssl.py	100.00% <100.00%> (ø)
astroid/brain/brain_typing.py	86.01% <100.00%> (-0.10%)	⬇️
astroid/brain/brain_unittest.py	100.00% <100.00%> (ø)
astroid/nodes/scoped_nodes/scoped_nodes.py	92.15% <100.00%> (ø)
astroid/rebuilder.py	96.60% <100.00%> (+5.37%)	⬆️