configs: lx2160a: Remove unused Flexspi config options

It is now safe to remove Flexspi configs NXP_FSPI_FLASH_SIZE and
NXP_FSPI_FLASH_NUM options from headers as they are no longer used by
driver anymore.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>