target-mips: get rid of MIPS_DEBUG

MIPS_DEBUG is a define used to dump the instruction disassembling. It
has to be defined at compile time. In practice I believe it's more
efficient to just look at the instruction disassembly and op dump using
-d in_asm,op. This patch therefore removes the corresponding code, which
clutters translate.c.

Cc: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>