gst/: Remove empty instance_init() functions to save relocs and lessen the noise...

gst/: Remove empty instance_init() functions to save relocs and lessen the noise. Remove some of the function prototy...

Original commit message from CVS:
* gst/gstindexfactory.c: (gst_index_factory_get_type):
* gst/gstplugin.c: (gst_plugin_init):
* gst/gstpluginfeature.c: (gst_plugin_feature_init):
* gst/gstquery.c: (gst_query_get_type):
* gst/gstregistry.c: (gst_registry_init):
* gst/gsturi.c: (gst_uri_handler_base_init):
Remove empty instance_init() functions to save relocs and lessen the
noise. Remove some of the function prototypes that are doubled by
G_DEFINE_TYPE.