defconfig: at91-sama5d2_ptc_ek: remove unused SYS_EXTRA_OPTIONS

Remove SYS_USE_NANDFLASH, SYS_USE_MMC as they are deprecated and
unused.
The board configurations already use CONFIG_SD_BOOT and
CONFIG_NAND_BOOT respectively.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[eugen.hristev@microchip.com: rework on latest u-boot]
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>