Terminate sortkey with prejudice

It has not been used by the search system since 2.8 and all code using
it in a meaningful way in Gerrit has been removed. Now we can remove
it from the database and API classes.

This requires a schema upgrade to delete the column and rebuild some
indexes to use last_updated_on instead of sort_key.

Change-Id: I87660f8e0e44bf11bafd9fa24f990351759df3fb

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeJson.java

@@ -379,7 +379,6 @@ public class ChangeJson {
     out.created = in.getCreatedOn();
     out.updated = in.getLastUpdatedOn();
     out._number = in.getId().get();
-    out._sortkey = in.getSortKey();
     out.starred = userProvider.get().getStarredChanges().contains(in.getId())
         ? true
         : null;