MIPS/opcodes: Remove DMFC3 and DMTC3 instructions

Coprocessor 3 has been removed from the MIPS ISA as from MIPS III[1][2]
with the LDC3 and SDC3 instructions having been replaced with LD and SD
instructions respectively and therefore the doubleword move instructions
from and to that coprocessor have never materialized (for 32-bit ISAs
coprocessor 3 has likewise been removed as from MIPS32r2[3]).  Remove
the DMFC3 and DMTC3 instructions from the opcode table then to avoid
confusion.

References:

[1] Charles Price, "MIPS IV Instruction Set", MIPS Technologies, Inc.,
    Revision 3.2, September, 1995, Section A 8.3.4 "Coprocessor 3 - COP3
    and CP3 load/store", p. A-176

[2] same, Table A-39 "CPU Instruction Encoding - MIPS III Architecture",
    p. A-179

[3] "MIPS32 Architecture For Programmers, Volume II: The MIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00086,
    Revision 2.00, June 9, 2003, Table A-2 "MIPS32 Encoding of the
    Opcode Field", p. 317

	opcodes/
	* mips-opc.c (mips_builtin_opcodes): Remove "dmfc3" and "dmtc3"
	entries and associated comments.

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