[CI] Drop CLEAR_CACHE Support in monolithic-* scripts
This patch drops support for clearing the cache with the CLEAR_CACHE
environment variable. This is an artifact of the old infrastructure as
we now do not persist the cache across builds, instead redownloading the
cache directory everytime. This makes the scripts slightly simpler as we
are no longer supporting unneeded functionality.
Reviewers: Endilll, cmtice, dschuff, Keenuts, lnihlen, gburgessiv
Reviewed By: Keenuts, cmtice
Pull Request: https://github.com/llvm/llvm-project/pull/149193
