Don't try to prune unless we're updating

Pruning is very expensive and we won't need it in the future
anyways.  This brings startup back to previous speeds (before
songvec changes).

src/song_save.c

@@ -150,5 +150,6 @@ void readSongInfoIntoList(FILE *fp, struct songvec *sv,
 	if (song)
 		insertSongIntoList(sv, song);
 
+	if (isUpdatingDB()) /* only needed until we get rid of forked update */
 		songvec_prune(sv);
 }