CI: stop testing for invalidations

[fingolfin]

This test is unreliable, often misleading, and we mostly ignore it anyway.

See also julia-actions/julia-invalidations#19

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.51%. Comparing base (48dbf93) to head (f914d2a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1569      +/-   ##
==========================================
- Coverage   75.69%   75.51%   -0.19%     
==========================================
  Files         357      357              
  Lines      113521   113521              
==========================================
- Hits        85930    85721     -209     
- Misses      27591    27800     +209     

see 27 files with indirect coverage changes