Remove fossabot in make gen/contributions

[t-kikuc]

What this PR does / why we need it:

Although we removed fossabot in #4945,
we need to remove it automatically when using make gen/contributoins.

Note

Even though the following sed -i won't work well on Mac, thatt doesn't matter.
That's because make gen/contributions will be used only by this GitHub Action(which uses ubuntu), not developers' PC.

# Remove fossabot from the contributors list.
sed -i '/fossabot/d' README.md.tmp

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.38%. Comparing base (b20163b) to head (35292c8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4960      +/-   ##
==========================================
- Coverage   29.38%   29.38%   -0.01%     
==========================================
  Files         322      322              
  Lines       40852    40852              
==========================================
- Hits        12006    12004       -2     
- Misses      27885    27888       +3     
+ Partials      961      960       -1     

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