Remove TargetMachine::adjustPassManager

Since opt no longer supports to run default (O0/O1/O2/O3/Os/Oz)
pipelines using the legacy PM, there are no in-tree uses of
TargetMachine::adjustPassManager remaining. This patch removes the
no longer used adjustPassManager functions.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D137796