[RISCV] Remove custom instruction selection for VFCVT_RM and friends (#72540)

We already have the pseudo's for lowering these as MI nodes with
rounding mode operands, and the generic FRM insertion pass. Doing the
insertion later in the backend allows SSA level passes to avoid
reasoning about physical register copies, and happens to produce better
code in practice. The later is mostly an accident of our insertion
order; we happen to place the frm write after the vsetvli, and it's very
common for a register to be killed at the vsetvli. End result is that we
get slightly better scalar register allocation.

I'm a bit unclear on the history here. I was surprised to find this code
in ISEL lowering at all, but am also surprised once I found it that all
the patterns and pseudos seem to already exist. My best guess is that
maybe we didn't do all the possible cleanup after introducing the
HasRoundMode mechanism?