mxs: Kconfig: Remove TARGET_XFI3 symbol

The xfi3 target has been removed by commit 539fba2c10e7 ("arm:
Remove xfi3 board"), but it missed to remove an entry from the
mxs Kconfig.

Remove it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/arch/arm/mach-imx/mxs/Kconfig b/arch/arm/mach-imx/mxs/Kconfig
index d3233d8..ccce6a7 100644
--- a/arch/arm/mach-imx/mxs/Kconfig
+++ b/arch/arm/mach-imx/mxs/Kconfig
@@ -18,9 +18,6 @@
 	select PL01X_SERIAL
 	select BOARD_EARLY_INIT_F
 
-config TARGET_XFI3
-	bool "Support xfi3"
-
 endchoice
 
 config SYS_SOC