[RISCV] Remove support for vfmv.v.f with bf16 type. (#95352)

This isn't used by clang and isn't in the rvv-intrinsic-doc.

The instruction requires Zvfh.

If the F register passed to the instruction isn't nan-boxed correctly,
the instruction will generate the wrong nan. So the instruction isn't a
generic move FPR16 to vector register instruction.