gst/autodetect/: Use gst_plugin_feature_list_free() to free feature list and in the case of autovideosink free the li...

Original commit message from CVS:
* gst/autodetect/gstautoaudiosink.c:
(gst_auto_audio_sink_find_best), (gst_auto_audio_sink_detect):
* gst/autodetect/gstautovideosink.c:
(gst_auto_video_sink_find_best), (gst_auto_video_sink_detect):
Use gst_plugin_feature_list_free() to free feature list and
in the case of autovideosink free the list at all. Also
miscellaneous cosmetic fixes.