gst/: Remove pre glib2.8 compatibility, fixes #340508

gst/: Remove pre glib2.8 compatibility, fixes #340508

Original commit message from CVS:
* gst/glib-compat.c:
* gst/gst.c: (init_pre):
* gst/gstobject.c: (gst_object_init), (gst_object_ref),
(gst_object_unref), (gst_object_replace), (gst_object_dispose),
(gst_object_dispatch_properties_changed):
* gst/gstobject.h:
* gst/gstregistryxml.c: (gst_registry_xml_read_cache):
* gst/gststructure.c: (gst_structure_set_valist):
* gst/gstvalue.c: (gst_date_get_type), (_gst_value_initialize):
Remove pre glib2.8 compatibility, fixes #340508