RISC-V: Reset the length to the default of 4 for FP comparisons

RISC-V: Reset the length to the default of 4 for FP comparisons

The default length for floating-point compare operations is overridden
to 8, however the FEQ.fmt, FLT.fmt, FLE.fmt machine instructions and
FGE.fmt, FGT.fmt assembly idioms the relevant RTL insns produce are all
4 bytes long each.  And all the floating-point compare RTL insns that
produce multiple machine instructions explicitly set their lengths.

Remove the override then, letting the default of 4 apply for the single
instruction case.

	gcc/
	* config/riscv/riscv.md (length): Remove the explicit setting
	for "fcmp".