MIPS: Remove remnants of 48-bit microMIPS instruction support

MIPS: Remove remnants of 48-bit microMIPS instruction support

The POOL48A major opcode was defined in early revisions of the 64-bit
microMIPS ISA, has never been implemented, and was removed before the
64-bit microMIPS ISA specification[1] has been finalized.

This complements commit a6c7053929dd ("MIPS/opcodes: Remove microMIPS
48-bit LI instruction").

References:

[1] "MIPS Architecture for Programmers, Volume II-B: The microMIPS64
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00594,
    Revision 3.06, October 17, 2012, Table 6.2 "microMIPS64 Encoding of
    Major Opcode Field", p. 578

	gas/
	* config/tc-mips.c (micromips_insn_length): Remove the mention
	of 48-bit microMIPS instructions.

	gdb/
	* mips-tdep.c (mips_insn_size): Remove 48-bit microMIPS
	instruction support.
	(micromips_next_pc): Likewise.
	(micromips_scan_prologue): Likewise.
	(micromips_deal_with_atomic_sequence): Likewise.
	(micromips_stack_frame_destroyed_p): Likewise.
	(mips_breakpoint_from_pc): Likewise.

	opcodes/
	* mips-dis.c (print_insn_micromips): Remove 48-bit microMIPS
	instruction support.