Remove dbus functionality if USE=dbus is not specified.

Index: xpra-3.0.2/xpra/scripts/server.py
===================================================================
--- xpra-3.0.2.orig/xpra/scripts/server.py
+++ xpra-3.0.2/xpra/scripts/server.py
@@ -748,14 +748,9 @@ def do_run_server(error_cb, opts, mode,
         log("chdir(%s)", opts.chdir)
         os.chdir(opts.chdir)
 
-    dbus_pid, dbus_env = 0, {}
     if not shadowing and POSIX and not OSX and not clobber:
         no_gtk()
         assert starting or starting_desktop or proxying
-        from xpra.server.dbus.dbus_start import start_dbus
-        dbus_pid, dbus_env = start_dbus(opts.dbus_launch)
-        if dbus_env:
-            os.environ.update(dbus_env)
 
     display = None
     if not proxying:
@@ -835,7 +830,6 @@ def do_run_server(error_cb, opts, mode,
         app.display_name = display_name
         app.init(opts)
         app.init_sockets(sockets)
-        app.init_dbus(dbus_pid, dbus_env)
         if not shadowing and (xvfb_pid or clobber):
             app.init_display_pid(xvfb_pid)
         app.original_desktop_display = desktop_display