[7/7][Clang][RISCV][NFC] Remove attribute `IsPrototypeDefaultTU`

This is the 7th commit of a patch-set that aims to remove the
IsPrototypeDefaultTU special case for the rvv-intrinsics.

This is the final commit of the patch-set. Now that no intrinsics is
using the attribute, we are safe to remove it.

Please refer to the cover letter in the 1st commit (D140895) for an
overview.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140954