[Core] Remove function to not override RoundTripPasses.

RoundTripPasses should always be called in DEBUG mode if the environment
variable "LLD_RUN_ROUNDTRIP_TEST" is set.

Flavors should not be able to override this behavior.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223073 91177308-0d34-0410-b5e6-96231b3b80d8