[MIPS] Remove commented out code to add -mmad for Nevada.

    
Adding -mmad is not usable since over half a decade in gcc and when
fixed the proper -march option values should enable the use of the
mad, madu and mul instructions of the R5500, RM5200, RM7000 and RM9000
families.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>