core-latest: Let gvfs install systemd files

Previously, gvfs always installed systemd files to the _system_ prefix
(/usr), but that has been updated to properly use the install prefix, so
this workaround is no longer needed.

See: GNOME/gvfs!124
2 jobs for diegoe_gvfs-systemd in 11 minutes and 40 seconds
latest
Status Job ID Name Coverage
  Test
passed test-py3 #1265317

00:01:32

passed ub20.04-py3-glib #1265318

00:11:40