[RISCV] Revert Zfhmin related changes that aren't tested and depend on f16 being a legal type.

Our Zfhmin support is only MC layer, but these are CodeGen layer
interfaces. If f16 isn't a Legal type for CodeGen with Zfhmin, then
these interfaces should keep their non-Zfh behavior.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D115822