Remove client_id/client_secret authentication

[s-t-e-v-e-n-k]

Authenticating via client_id and client_secret is dangerous, since they
appended to the query string, and can be easily seen from the URL.

client_id/client_secret have been deprecated since May 2020, and ignored
by GitHub since late 2020, so it's high time they were removed.

[codecov-io]

Codecov Report

Merging #1888 (d054984) into master (ddd437a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1888      +/-   ##
==========================================
- Coverage   98.88%   98.88%   -0.01%     
==========================================
  Files         108      108              
  Lines       11052    11042      -10     
==========================================
- Hits        10929    10919      -10     
  Misses        123      123              

Impacted Files	Coverage Δ
.../runner/work/PyGithub/PyGithub/github/MainClass.py	95.56% <0.00%> (-0.08%)	⬇️
.../runner/work/PyGithub/PyGithub/github/Requester.py	98.44% <0.00%> (-0.03%)	⬇️

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