Remove -simplify-libcalls pass form Passes documentation

This pass was removed in r184459.

Also added note that the InstCombine pass does library call
simplification.

Patch slightly modified from one by Daniel Liew
<daniel.liew@imperial.ac.uk>!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206650 91177308-0d34-0410-b5e6-96231b3b80d8