Remove regtest.pl and regtest.py from TrigValTools

The functionality of these two scripts had been moved to TrigValTools/TrigValSteering/CheckSteps.py => RegTestStep to a large extend already and the missing "diff" has now been also merged into RegTestStep. The two scripts are therefor not needed any longer.

cc @fwinkl , @hartj , @smh , @okumura

Closes ATR-28931