RISC-V: Remove FRM for vfwcvt (RVV float to float widening conversion)

Base on the discussion here:
https://github.com/riscv/riscv-v-spec/issues/884



vfwcvt doesn't depend on FRM. So remove FRM preparing for mode switching support.

gcc/ChangeLog:

	* config/riscv/vector.md: Remove FRM.

Signed-off-by: Pan Li <pan2.li@intel.com>