target-mips: get rid of MIPS_DEBUG_SIGN_EXTENSIONS

MIPS_DEBUG_SIGN_EXTENSIONS was used sometimes ago to verify that 32-bit
instructions correctly sign extend their results. It's now not need
anymore, remove it.

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>
