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/change/ConsistencyChecker.java

@@ -583,8 +583,6 @@ public class ConsistencyChecker {
         // historical information.
         db.accountPatchReviews().delete(
             db.accountPatchReviews().byPatchSet(psId));
-        db.patchSetAncestors().delete(
-            db.patchSetAncestors().byPatchSet(psId));
         db.patchSetApprovals().delete(
             db.patchSetApprovals().byPatchSet(psId));
         db.patchComments().delete(