[ci] Remove redundant main.cpp and avoid specifying C++ standard.

[vgvassilev]

This should help us build with newer gtest versions which require C++ later than 11.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.67%. Comparing base (df17170) to head (e3715be).
Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1253   +/-   ##
=======================================
  Coverage   94.66%   94.67%           
=======================================
  Files          51       51           
  Lines        8886     8919   +33     
=======================================
+ Hits         8412     8444   +32     
- Misses        474      475    +1     

see 6 files with indirect coverage changes

see 6 files with indirect coverage changes