libs/gst/net/: Massive code removal and cleanups because of GstPoll.

libs/gst/net/: Massive code removal and cleanups because of GstPoll.

Original commit message from CVS:
Patch by: Peter Kjellerstedt <pkj at axis dot com>
* libs/gst/net/gstnetclientclock.c: (gst_net_client_clock_init),
(gst_net_client_clock_finalize), (gst_net_client_clock_do_select),
(gst_net_client_clock_thread), (gst_net_client_clock_start),
(gst_net_client_clock_stop), (gst_net_client_clock_new):
* libs/gst/net/gstnetclientclock.h:
* libs/gst/net/gstnettimeprovider.c: (gst_net_time_provider_init),
(gst_net_time_provider_finalize), (gst_net_time_provider_thread),
(gst_net_time_provider_start), (gst_net_time_provider_stop),
(gst_net_time_provider_new):
* libs/gst/net/gstnettimeprovider.h:
Massive code removal and cleanups because of GstPoll.
Fixes #505417.