Revert "[Plugins] Add a slim plugin API to work together with the new PM"

This reverts commit ecf3ba1ab45edb1b0fadce716a7facf50dca4fbb/r329273.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329276 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed