Revert "Permit dump/reload of not-too-large >1GB tuples"

This reverts commits fa2fa99552 and 42f50cb8fa.

While the functionality that was intended to be provided by these
commits is desired, the patch didn't actually solve as many of the
problematic situations as we hoped, and it created a bunch of its own
problems.  Since we're going to require more extensive changes soon for
other reasons and users have been working around these problems for a
long time already, there is no point in spending effort in fixing this
halfway measure.

Per complaint from Tom Lane.
Discussion: https://postgr.es/m/21407.1484606922@sss.pgh.pa.us

(Commit fa2fa99552 had already been reverted in branches 9.5 as
f858524ee4 and 9.6 as e9e44a0953, so this touches master only.
Commit 42f50cb8fa was not present in the older branches.)

src/backend/access/common/heaptuple.c

@@ -741,9 +741,7 @@ heap_form_tuple(TupleDesc tupleDescriptor,
 	 * Allocate and zero the space needed.  Note that the tuple body and
 	 * HeapTupleData management structure are allocated in one chunk.
 	 */
-	tuple = MemoryContextAllocExtended(CurrentMemoryContext,
-									   HEAPTUPLESIZE + len,
-									   MCXT_ALLOC_HUGE | MCXT_ALLOC_ZERO);
+	tuple = (HeapTuple) palloc0(HEAPTUPLESIZE + len);
 	tuple->t_data = td = (HeapTupleHeader) ((char *) tuple + HEAPTUPLESIZE);
 
 	/*