[RISCV] Remove pseudos for vwcvt.f.x(u) with rounding mode.

vwcvt.f.x doesn't use rounding mode. The integer value fits in
the mantissa of a 2x larger FP type so no rounding is required.

I've remove the Uses = [FRM] that is also not needed.

I deleted the isel patterns. Alternatively, we could keep them and
drop the rounding mode immediate. The patterns are currently untested
so I chose to delete them. If they become needed in the future, we
can decide then if we should have the patterns or teach the node
creation to use the non-RM form for widening.

This reverts part of D142102.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D154653