lib-index: mail_index_sync_map() - Don't try to-resync extension updates

This was done to call extension record sync handlers, but the previous
commit removes them. Fixes a problem where obsolete cache offsets were
used in some situations:

 - Some cache updates are from external transactions and some are from
non-external transactions. This is because cache offset updates are being
added by whatever the parent index transaction is.

 - When mail_index_sync_map() is mapping MAIL_INDEX_SYNC_HANDLER_FILE, it
has already synced the map. But it's calling mail_index_sync_record()
for non-external transactions to call expunge handlers and extension
update handlers. It's calling the regular mail_index_sync_record() to do
this work.

 - But mail_index_sync_record() is actually still updating the map. So now
mail_index_sync_record() is called for all non-external cache updates,
but not for external cache updates! And since these are somewhat
randomly either external or non-external, the end result is that the
cache offset may be obsolete.

src/lib-index/mail-index-sync-update.c

@@ -1028,14 +1028,9 @@ int mail_index_sync_map(struct mail_index_map **_map,
 		if (LOG_IS_BEFORE(prev_seq, prev_offset,
 				  view->map->hdr.log_file_seq,
 				  view->map->hdr.log_file_head_offset)) {
-			/* this has been synced already. we're here only to call
-			   expunge handlers and extension update handlers. */
+			/* this has been synced already. */
 			i_assert(type == MAIL_INDEX_SYNC_HANDLER_FILE);
-
-			if ((thdr->type & MAIL_TRANSACTION_EXTERNAL) != 0)
-				continue;
-			if ((thdr->type & MAIL_TRANSACTION_EXT_MASK) == 0)
-				continue;
+			continue;
 		}
 
 		/* we'll just skip over broken entries */