target/mips: Use decode_ase_msa() generated from decodetree

Now that we can decode the MSA ASE with decode_ase_msa(),
use it and remove the previous code, now unreachable.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201215225757.764263-21-f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>