package/gcc: remove BR2_GCC_SUPPORTS_FINEGRAINEDMTUNE

Since commit 8f8e9162 ("package/gcc:
do not mourn avr32 for too long..."), in which we dropped AVR32
support, the BR2_GCC_SUPPORTS_FINEGRAINEDMTUNE is always
'y'. Therefore, it is totally useless to keep this option around, and
this commit removes it, making the corresponding code unconditional
along the way.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>