[llvm-rtdyld] Don't waste cycles invalidating instruction cache.

Now that setExecutable() changed to do all the ground work to make
memory executable on the host, we can remove all (redundant) calls
to invalidate instruction cache here.

As an added bonus, this makes invalidateInstructionCache() dead
code, so it can be removed.

Differential Revision:	http://reviews.llvm.org/D13631

llvm-svn: 253343