Remove belt-and-suspenders guards against buffer pin leaks.

Forcibly releasing all leftover buffer pins should be unnecessary now
that we have a robust ResourceOwner mechanism, and it significantly
increases the cost of process shutdown.  Instead, in an assert-enabled
build, assert that no pins are held; in a non-assert-enabled build, do
nothing.

src/backend/storage/buffer/bufmgr.c

@@ -1659,31 +1659,26 @@ InitBufferPoolBackend(void)
 }
 
 /*
- * Ensure we have released all shared-buffer locks and pins during backend exit
+ * During backend exit, ensure that we released all shared-buffer locks and
+ * assert that we have no remaining pins.
  */
 static void
 AtProcExit_Buffers(int code, Datum arg)
 {
-	int			i;
-
 	AbortBufferIO();
 	UnlockBuffers();
 
-	for (i = 0; i < NBuffers; i++)
+#ifdef USE_ASSERT_CHECKING
+	if (assert_enabled)
 	{
-		if (PrivateRefCount[i] != 0)
-		{
-			volatile BufferDesc *buf = &(BufferDescriptors[i]);
+		int			i;
 
-			/*
-			 * We don't worry about updating ResourceOwner; if we even got
-			 * here, it suggests that ResourceOwners are messed up.
-			 */
-			PrivateRefCount[i] = 1;		/* make sure we release shared pin */
-			UnpinBuffer(buf, false);
+		for (i = 0; i < NBuffers; i++)
+		{
 			Assert(PrivateRefCount[i] == 0);
 		}
 	}
+#endif
 
 	/* localbuf.c needs a chance too */
 	AtProcExit_LocalBuffers();

src/backend/storage/buffer/localbuf.c

@@ -468,14 +468,23 @@ AtEOXact_LocalBuffers(bool isCommit)
 /*
  * AtProcExit_LocalBuffers - ensure we have dropped pins during backend exit.
  *
- * This is just like AtProcExit_Buffers, but for local buffers.  We have
- * to drop pins to ensure that any attempt to drop temp files doesn't
- * fail in DropRelFileNodeBuffers.
+ * This is just like AtProcExit_Buffers, but for local buffers.  We shouldn't
+ * be holding any remaining pins; if we are, and assertions aren't enabled,
+ * we'll fail later in DropRelFileNodeBuffers while trying to drop the temp
+ * rels.
  */
 void
 AtProcExit_LocalBuffers(void)
 {
-	/* just zero the refcounts ... */
-	if (LocalRefCount)
-		MemSet(LocalRefCount, 0, NLocBuffer * sizeof(*LocalRefCount));
+#ifdef USE_ASSERT_CHECKING
+	if (assert_enabled && LocalRefCount)
+	{
+		int			i;
+
+		for (i = 0; i < NLocBuffer; i++)
+		{
+			Assert(LocalRefCount[i] == 0);
+		}
+	}
+#endif
 }