CI: Remove the CI for cargo tarpaulin + Coveralls

[tatsuya6502]

Remove the CI running on Circle CI for cargo tarpaulin + Coveralls. The code coverage tool was already replaced by cargo llvm-cov + Codecov via #429.

Also hide the FOSSA status from the README. The FOSSA job is not working correctly; it is emitting error "no dependencies found". https://app.fossa.com/projects/git%2Bgit.luolix.top%2Fmoka-rs%2Fmoka/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.97%. Comparing base (c78027a) to head (5f19d5f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
+ Coverage   94.94%   94.97%   +0.03%     
==========================================
  Files          44       44              
  Lines       20983    20983              
==========================================
+ Hits        19922    19929       +7     
+ Misses       1061     1054       -7     

[tatsuya6502]

Merging.