configs/beaglebone_qt5_defconfig: fix kernel build

In commit 4b2440b7 ("beaglebone: add DT
for BeagleBone Green"), both beaglebone_defconfig and
beaglebone_qt5_defconfig were updated to add "am335x-bonegreen" to the
list of Device Tree files to be produced.

However, beaglebone_qt5_defconfig uses an older kernel version than
beaglebone_defconfig, in which am335x-bonegreen doesn't exist.

Therefore, revert the change on beaglebone_qt5_defconfig.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/builds/15762182



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>