target/mips: Convert Rel6 LWL/LWR/SWL/SWR opcodes to decodetree

LWL/LWR/SWL/SWR opcodes have been removed from the Release 6.

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

Remove unreachable check_insn_opc_removed() calls.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201208203704.243704-10-f4bug@amsat.org>