Remove old git clean command

Description

In !1281 (merged) we changed how we run git clean on the users repository, to allow for GIT_CLEAN_FLAGs(https://docs.gitlab.com/ee/ci/large_repositories/#git-clean-flags). Just in case we introduce a breaking change for the users CI, we have introduced FF_USE_LEGACY_GIT_CLEAN_STRATEGY so the user can use the legacy strategy until they move their build script accordingly

Proposal

Remove the old code path and the FF_USE_LEGACY_GIT_CLEAN_STRATEGY feature flag

Links to related issues and merge requests / references

Main deprecation issue #3473 (closed)