arm: at91: drop machine defconfig

No one care about them for releases and the machine are present in soc
defconfigs so drop them.

For qil-a9260 and usb-a9260 I maintain them and now switch to the
at91sam9260_defconfig.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>