Revert "[TLI] Per-function fveclib for math library used for vectorization"

This reverts commit 60c642e7.

This patch is making the TLI "closed" for a predefined set of VecLib
while at the moment it is extensible for anyone to customize when using
LLVM as a library.
Reverting while we figure out a way to re-land it without losing the
generality of the current API.

Differential Revision: https://reviews.llvm.org/D77925