Remove PatchSetAncestors table

The only remaining callers are the insert and delete code paths; it is
no longer used for lookups.

Change-Id: Id665eda39b585750f1b4032312ea04f5539456b4

gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java

@@ -2252,7 +2252,6 @@ public class ReceiveCommits {
           return null;
         }
 
-        ChangeUtil.insertAncestors(db, newPatchSet.getId(), newCommit);
         if (newPatchSet.getGroups() == null) {
           newPatchSet.setGroups(GroupCollector.getCurrentGroups(db, change));
         }