graphics/gpu-firmware-*-kmod: Remove defualt FLAVOR

The default flavor is always the first of the list.
Setting one make the port framework always use this one and so
things like make clean will never clean all the flavors.
This will lead to leave all the work-${FLAVOR} directories except
for the default one if one is building graphics/gpu-firmware-kmod.

PR: 263899
Sponsored by: Beckhoff Automation GmbH & Co. KG