Drop APIs deprecated in 2.0

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• It's time to clean up!

[codecov]

Codecov Report

Merging #5370 into master will increase coverage by 0.48%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #5370      +/-   ##
==========================================
+ Coverage    82.1%   82.59%   +0.48%     
==========================================
  Files         303      286      -17     
  Lines       40165    39761     -404     
  Branches     6206     6145      -61     
==========================================
- Hits        32977    32840     -137     
+ Misses       5820     5561     -259     
+ Partials     1368     1360       -8

Impacted Files	Coverage Δ
sphinx/testing/util.py	92.79% <ø> (-0.13%)	⬇️
sphinx/transforms/post_transforms/__init__.py	65.97% <ø> (+5.47%)	⬆️
sphinx/ext/intersphinx.py	86.78% <ø> (+0.57%)	⬆️
sphinx/builders/__init__.py	78.5% <ø> (+0.82%)	⬆️
sphinx/application.py	75.07% <ø> (+2.56%)	⬆️
setup.py	0% <ø> (ø)	⬆️
sphinx/parsers.py	77.41% <ø> (+4.69%)	⬆️
sphinx/domains/cpp.py	83.83% <0%> (ø)	⬆️
sphinx/theming.py	73.24% <100%> (+4.82%)	⬆️
sphinx/deprecation.py	62.06% <100%> (-2.45%)	⬇️
... and 18 more

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