target/mips: Convert Rel6 COP1X opcode to decodetree

COP1x opcode has been removed from the Release 6.

Add a single decodetree entry for it, triggering
Reserved Instruction if ever used.

Remove unreachable check_insn_opc_removed() call.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201208203704.243704-8-f4bug@amsat.org>

target/mips/mips32r6.decode

@@ -16,4 +16,6 @@
 
 LSA                 000000 ..... ..... ..... 000 .. 000101  @lsa
 
+REMOVED             010011 ----- ----- ----- ----- ------   # COP1X (COP3)
+
 REMOVED             011100 ----- ----- ----- ----- ------   # SPECIAL2

target/mips/translate.c

@@ -28826,7 +28826,6 @@ static bool decode_opc_legacy(CPUMIPSState *env, DisasContext *ctx)
         break;
     case OPC_CP3:
-        check_insn_opc_removed(ctx, ISA_MIPS_R6);
         if (ctx->CP0_Config1 & (1 << CP0C1_FP)) {
             check_cp1_enabled(ctx);
             op1 = MASK_CP3(ctx->opcode);