configs/freescale_imx6ulevk: Remove target

configs/freescale_imx6ulevk: Remove target

Currently this target fails to build:

./scripts/dtc-version.sh: line 17: dtc: command not found
./scripts/dtc-version.sh: line 18: dtc: command not found
*** Your dtc is too old, please upgrade to dtc 1.4 or newer

Even though the fix for this build failure is easy, I prefer to
maintain only imx6ulevk_defconfig, which is a target for the
same board, but uses mainline kernel and U-Boot versions instead.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/66561695
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>