tbs2910: remove SYS_SOC from board specific Kconfig

SYS_SOC is already defined in arch/arm/cpu/armv7/mx6/Kconfig, no need to
define it again

Signed-off-by: Soeren Moch <smoch@web.de>

board/tbs/tbs2910/Kconfig

@@ -6,9 +6,6 @@ config SYS_BOARD
 config SYS_VENDOR
 	default "tbs"
 
-config SYS_SOC
-	default "mx6"
-
 config SYS_CONFIG_NAME
 	default "tbs2910"