ApeTag: drop support for non-standard tag "album artist"

This was used by proprietary software.  MPD adopted it a few years
ago, which turns out to be a mistake, because it now creates problems
for some MPD users (http://bugs.musicpd.org/view.php?id=4168).

src/tag/ApeTag.cxx

@@ -30,7 +30,6 @@
 #include <string.h>
 
 const struct tag_table ape_tags[] = {
-	{ "album artist", TAG_ALBUM_ARTIST },
 	{ "year", TAG_DATE },
 	{ nullptr, TAG_NUM_OF_ITEM_TYPES }
 };