configs/freescale_imx6qsabresd_defconfig: fix defconfig

While switching ATF to github, the BR2_TARGET_UBOOT_CUSTOM_GIT=y
symbol was not removed. Since then this defconfig fail to build
in gitlab-ci due to invalid defconfig check.

  WARN: defconfig ./configs/freescale_imx6qsabresd_defconfig can't be used:
        Missing: BR2_TARGET_UBOOT_CUSTOM_GIT=y

[1] dd42b159a5f752921ee602530d289c810ad7ee51

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4889436612

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 0ee8ef970bab8680162175d127e388a04286791f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/freescale_imx6qsabresd_defconfig

@@ -37,7 +37,6 @@ BR2_PACKAGE_HOST_MTOOLS=y
 BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BOARDNAME="mx6qsabresd"
 BR2_TARGET_UBOOT_FORMAT_IMX=y
-BR2_TARGET_UBOOT_CUSTOM_GIT=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="$(call github,nxp-imx,uboot-imx,lf-5.10.y-1.0.0)/uboot-imx-lf-5.10.y-1.0.0.tar.gz"
 BR2_TARGET_UBOOT_NEEDS_DTC=y