configs/nitrogen6x_defconfig: remove duplicate BR2_PACKAGE_HOST_UBOOT_TOOLS=y

The commit [1] added this option a second time.
Remove the first occurence.

Fixes:
configs/nitrogen6x_defconfig:31:warning: override: reassigning to symbol BR2_PACKAGE_HOST_UBOOT_TOOLS

[1] 6ea9f662a03e0f4b4a9000a25095b8e6293b07b6

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 667a68c4a6a51287330f56c235d7f3a28c00da3a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/nitrogen6x_defconfig

@@ -11,7 +11,6 @@ BR2_TARGET_GENERIC_GETTY_PORT="ttymxc1"
 BR2_ROOTFS_POST_BUILD_SCRIPT="board/boundarydevices/common/post-build.sh"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="board/boundarydevices/common/post-image.sh"
 BR2_PACKAGE_HOST_GENIMAGE=y
-BR2_PACKAGE_HOST_UBOOT_TOOLS=y
 
 # filesystem
 BR2_TARGET_ROOTFS_EXT2=y