[libFuzzer] Remove entropic-scale-per-exec-time.test.

The test has been flaky for years, and I think we should remove it to
eliminate noise on the buildbot.

Neither me nor dokyungs have been able to fully deflake the test, and it
tests a non-default Entropic flag.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D115453