[RISCV] Remove ForceTailAgnostic flag from vmv.s.x, vfmv.s.f and reductions.

In 0.9 these were defined to leave elements other than 0 in the
destination unmodified. They were changed to use the tail policy
in 0.10. I missed that update.

I assume no one has noticed because in order cores treat tail
agnostic the same as tail undisturbed. I believe Spike and QEMU do
the same.

Reviewed By: arcbbb, frasercrmck

Differential Revision: https://reviews.llvm.org/D103736