AVR32: Remove useless config option "GENERIC_BUST_SPINLOCK".

Remove the clearly useless config option GENERIC_BUST_SPINLOCK, which
is not used anywhere in the tree.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>