[IR] Remove some unused #includes (NFC)

I needed a reader-writer lock for a downstream project and noticed that
llvm has one. Function.cpp is the only file in-tree that refers to it.
To anyone reading this: are you using RWMutex in out-of-tree code? Maybe
it's not worth keeping around any more...

Since we're not actually using RWMutex *here*, remove the #include (and
a few other stale headers while we're at it).

llvm-svn: 278178