arm: sam9_l9260_defconfig: remove non-existing config option

CONFIG_MTD_UBI_BEB_RESERVE has been removed and now we use
CONFIG_MTD_UBI_BEB_LIMIT instead.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>