[db] Remove some definitions of ARTWORK_XXX

The artwork db field should only be used to save media file artwork metadata,
which mostly means whether it has embedded artwork or not. It should not be
used to save where we found artwork for the file, since that is not static.

So this commit removes for instance ARTWORK_OWN, ARTWORK_DIR etc., which we
weren't using anyway.

src/library/filescanner_ffmpeg.c

@@ -393,7 +393,6 @@ scan_metadata_ffmpeg(struct media_file_info *mfi, const char *file)
 	return -1;
 
       av_dict_set(&options, "icy", "1", 0);
-      mfi->artwork = ARTWORK_HTTP;
     }
 
   ret = avformat_open_input(&ctx, path, NULL, &options);