TrigAnalysisTest: Add AthFile cache clearing step in trigRel21 tests
Add a step removing the athfile-cache.ascii.gz
file in tests where the Trigger step runs in release 21.0. These files produced in 21.0 are incompatible with python3 r22 and loading them in the downstream job leads to a SyntaxError. Simply removing them should avoid the problem.
Fixes ATR-21489