Revert "Use Foreign Key relationships to infer multi-column join selectivity".

This commit reverts 137805f as well as the associated commits 015e889,
5306df2, and 68d704e.  We found multiple bugs in this feature, and
there was concern about possible planner slowdown (though to be fair,
exhibiting a very large slowdown proved difficult).  The way forward
requires a considerable rewrite, which may or may not be possible to
accomplish in time for beta2.  In my judgment reviewing the rewrite will
be easier to accomplish starting from a clean slate, so let's temporarily
revert what's there now.  This also leaves us in a safe state if it turns
out to be necessary to postpone the rewrite to the next development cycle.

Discussion: <20160429102531.GA13701@huehner.biz>

Author: tglsfdc

src/backend/nodes/outfuncs.c

@@ -2137,16 +2137,6 @@ _outIndexOptInfo(StringInfo str, const IndexOptInfo *node)
 	/* we don't bother with fields copied from the index AM's API struct */
 }
 
-static void
-_outForeignKeyOptInfo(StringInfo str, const ForeignKeyOptInfo *node)
-{
-	WRITE_NODE_TYPE("FOREIGNKEYOPTINFO");
-
-	WRITE_OID_FIELD(conrelid);
-	WRITE_OID_FIELD(confrelid);
-	WRITE_INT_FIELD(nkeys);
-}
-
 static void
 _outEquivalenceClass(StringInfo str, const EquivalenceClass *node)
 {
@@ -3617,9 +3607,6 @@ outNode(StringInfo str, const void *obj)
 			case T_IndexOptInfo:
 				_outIndexOptInfo(str, obj);
 				break;
-			case T_ForeignKeyOptInfo:
-				_outForeignKeyOptInfo(str, obj);
-				break;
 			case T_EquivalenceClass:
 				_outEquivalenceClass(str, obj);
 				break;