This reverts commit c37cd19f.

Now that we've reverted the commit "gtestdbus: Properly close server
connections", g_test_dbus_down() no longer returns early and we no
longer need this workaround. Since the gdbus-names test seems to
properly unref its GDBusConnection objects it's not clear to me why it
needed the sleep to succeed. However even at the time the failure wasn't
reproducible according to this comment[1] so it's probably not worth
spending more effort trying to reproduce it now.

[1] #787 (comment 214235)