MIPS/opcodes: Discard unused OP_SH, OP_MASK, and OP_OP macros

As from commit ab902481 ("Add structures to describe MIPS
operands"), <https://sourceware.org/ml/binutils/2013-07/msg00135.html>,
the use of numerous regular MIPS and microMIPS OP_SH and OP_MASK macros
has been removed.

Similarly as from commit c3c07478 ("Use operand structures for
MIPS16"), <https://sourceware.org/ml/binutils/2013-07/msg00136.html>,
the use of numerous MIPS16 OP_SH and OP_MASK macros has been removed.

And as from commit 9e12b7a2 ("Rewrite main mips_ip parsing loop"),
<https://sourceware.org/ml/binutils/2013-07/msg00139.html>, none of the
OP_OP macros are used anymore.

Discard all the unused macros then and only keep the small subset that
is still referred.  This simplifies maintenance and removes the need to
keep the artificial arrangement where some regular MIPS and microMIPS
macros expand to 0 and are kept for compatibility with the opposite ISA
mode only, as it used to be required before the commit referred.