imx: mx6ul_14x14_evk discard redefined CONFIG_MX6

Discard CONFIG_MX6 in mx6ul_14x14_evk.h, since it is already defined
in mx6_common.h.

Tested on mx6ul_14x14_evk board.
Signed-off-by: NPeng Fan <Peng.Fan@freescale.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: NStefano Babic <sbabic@denx.de>

include/configs/mx6ul_14x14_evk.h

@@ -20,7 +20,6 @@
 #define CONFIG_SPL_FAT_SUPPORT
 #include "imx6_spl.h"
 
-#define CONFIG_MX6
 #define CONFIG_ROM_UNIFIED_SECTIONS
 #define CONFIG_SYS_GENERIC_BOARD
 #define CONFIG_DISPLAY_CPUINFO