refactor: sphinx.util.inspect:Signature.format_annotation_old()

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• I have a plan to separate format_annotation() methods to independent function.
• Before the separation, this refactors them to remove meaningless codes.

Note: I tested this locally with py3.5.1, 3.5.2, 3.5.7 and 3.6.9.

[codecov]

Codecov Report

Merging #6580 into 2.0 will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##              2.0    #6580      +/-   ##
==========================================
- Coverage   83.79%   83.76%   -0.03%     
==========================================
  Files         266      268       +2     
  Lines       40834    40868      +34     
  Branches     5982     5985       +3     
==========================================
+ Hits        34217    34234      +17     
- Misses       5295     5309      +14     
- Partials     1322     1325       +3

Impacted Files	Coverage Δ
sphinx/util/inspect.py	72.43% <0%> (ø)	⬆️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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