[AArch64] Don't map llvm sqrt intrinsics to veclib functions

Since AArch64 has sqrt instructions, we want to use those instead of
calls to vector math routines for llvm sqrt intrinsics (since those
don't imply some of the constraints that libm calls might have) so
we just remove the mappings.

Code originally written by mgabka

Reviewed By: danielkiss, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D145392