Remove unnecessary files from releng's airootfs

configs/releng/airootfs/etc/fstab:
/etc/fstab shipped by the filesystem package has only comments. There is no reason to replace it with an empty file.

configs/releng/airootfs/etc/systemd/system/default.target:
There is no harm in booting to graphical.target. releng does not enable (or even install) any service that has {Required,Wanted}By=graphical.target.