configs/wandboard: Remove BR2_LINUX_KERNEL_CUSTOM_GIT

Now that we use a mainline kernel for wandboard, we no longer
need to select BR2_LINUX_KERNEL_CUSTOM_GIT, so remove it.

This fixes the following warning:

make wandboard_defconfig
/configs/wandboard_defconfig:19:warning: override: BR2_LINUX_KERNEL_CUSTOM_VERSION changes choice state
#
# configuration written to /home/fabio/buildroot/.config

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

configs/wandboard_defconfig

@@ -15,7 +15,6 @@ BR2_TARGET_UBOOT_FORMAT_IMG=y
 BR2_TARGET_UBOOT_SPL=y
 BR2_TARGET_UBOOT_SPL_NAME="SPL"
 BR2_LINUX_KERNEL=y
-BR2_LINUX_KERNEL_CUSTOM_GIT=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.7.3"
 BR2_LINUX_KERNEL_DEFCONFIG="imx_v6_v7"