Remove the simplify-libcalls pass (finally)

This commit completely removes what is left of the simplify-libcalls
pass.  All of the functionality has now been migrated to the instcombine
and functionattrs passes.  The following C API functions are now NOPs:

  1. LLVMAddSimplifyLibCallsPass
  2. LLVMPassManagerBuilderSetDisableSimplifyLibCalls

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184459 91177308-0d34-0410-b5e6-96231b3b80d8