dbind: remove call to atspi_dbus_connection_setup_with_g_main

dbind isn't currently running a GMainLoop, so it shouldn't call
atspi_dbus_connection_setup_with_g_main, and calling this is harmful if
it is being called from a context other than the default context, since
D-Bus messages will no longer be delivered.

https://bugzilla.gnome.org/show_bug.cgi?id=707218