[4/15][Clang][RISCV][NFC] Remove unnecessary logic under RVVIntrinsic::computeBuiltinTypes

`PolicyAttrs::IsUnspecified` is modified in
`RVV::Intrinsic::computeBuiltInTypes` and used under
`RVVIntrinsic::updateNamesAndPolicy`. Suprisingly the modification
in `RVV::Intrinsic::computeBuiltInTypes` is completely unnecessary.
This commit removes the redundant logic.

This is the 4th 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

Differential Revision: https://reviews.llvm.org/D141577