[11/15][Clang][RISCV][NFC] Remove Policy::PolicyType::Omit

The attribute can be removed now as preceding patches have removed its
users.

This is the 11th commit of a patch-set that aims to change the default policy
for RVV intrinsics from TAMU to TAMA.

Please refer to the cover letter in the 1st commit (D141573) for an
overview.

Reviewed By: craig.topper, kito-cheng

Differential Revision: https://reviews.llvm.org/D141768