audioconvert: don't use .init function

Don't use the .init function but compile all functions when needed instead of
when the plugin is registered.

gst/audioconvert/gstaudioconvertorc.orc

@@ -1,6 +1,4 @@
 
-.init gst_audio_convert_orc_init
-
 .function orc_audio_convert_unpack_u8
 .dest 4 d1 gint32
 .source 1 s1 guint8

gst/audioconvert/plugin.c

@@ -31,8 +31,6 @@
 static gboolean
 plugin_init (GstPlugin * plugin)
 {
-  gst_audio_convert_orc_init ();
-
   /* ensure GstAudioChannelPosition type is registered */
   if (!gst_audio_channel_position_get_type ())
     return FALSE;