s390: Remove s390_format_RR and friends.

s390: Remove s390_format_RR and friends.

This patch removes the functions: s390_format_RR, s390_format_RR_RR
and s390_format_RR_FF.

First, because of the disassembly changes in 370531396ad there is no
longer a need to specialise s390_format_RR because the register class
of its operands is now irrelevant. Hence s390_format_RR_RR and
s390_format_RR_FF can go.
Secondly, s390_irgen_RR only calls the function pointer being passed in.
So we can call that function directly and s390_irgen_RR can go as well.

The same reasoning also applies to other opcode formats. Patches will
follow soon. Having several installments gives a chance to actually
proofread them.