Remove obsolete "importorskip('unittest.mock')" calls

[nicoddemus]

No longer make sense given that unittest.mock is part of the stdlib for all Python versions we support now.

[codecov]

Codecov Report

Merging #5608 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5608      +/-   ##
==========================================
+ Coverage   96.11%   96.12%   +0.01%     
==========================================
  Files         117      117              
  Lines       25775    25770       -5     
  Branches     2495     2495              
==========================================
- Hits        24773    24771       -2     
+ Misses        697      695       -2     
+ Partials      305      304       -1

Impacted Files	Coverage Δ
testing/python/integration.py	94.07% <100%> (+0.5%)	⬆️
src/_pytest/pastebin.py	91.22% <0%> (+3.5%)	⬆️

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