target/mips: fix delay slot detection in gen_msa_branch()

It is unnecessary to test R6 from delay/forbidden slot check
in gen_msa_branch().

https://bugs.launchpad.net/qemu/+bug/1663287

Reported-by: Brian Campbell <bacam@z273.org.uk>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
