mx5: Fix CONFIG_OF_LIBFDT redefined warning

With the following commit, CONFIG_OF_LIBFDT is redefined.

  2fa8ca98c37d5b1bb0328b19ddb7e9d162cd9683
  Add CONFIG_OF_LIBFDT to more boards.

Remove the duplicated definition to fix CONFIG_OF_LIBFDT redefined
warning.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

include/configs/mx51evk.h

@@ -226,6 +226,4 @@
 #define CONFIG_ENV_IS_IN_MMC
 #define CONFIG_SYS_MMC_ENV_DEV 0
 
-#define CONFIG_OF_LIBFDT
-
 #endif

include/configs/mx53evk.h

@@ -192,6 +192,4 @@
 #define CONFIG_ENV_IS_IN_MMC
 #define CONFIG_SYS_MMC_ENV_DEV 0
 
-#define CONFIG_OF_LIBFDT
-
 #endif				/* __CONFIG_H */