Revert "graceful shutdown" changes for Windows.

This reverts commits 6051857fc and ed52c3707 in HEAD (they were already
reverted in the back branches).  Further testing has shown that while
those changes do fix some things, they also break others; in particular,
it looks like walreceivers fail to detect walsender-initiated connection
close reliably if the walsender shuts down this way.  A proper fix for
this seems possible but won't be done in time for v15.

Discussion: https://postgr.es/m/CA+hUKG+OeoETZQ=Qw5Ub5h3tmwQhBmDA=nuNO3KG=zWfUypFAw@mail.gmail.com
Discussion: https://postgr.es/m/CA+hUKGKkp2XkvSe9nG+bsgkXVKCdTeGSa_TR0Qx1jafc_oqCVA@mail.gmail.com

src/backend/libpq/pqcomm.c

@@ -277,30 +277,15 @@ socket_close(int code, Datum arg)
 		secure_close(MyProcPort);
 
 		/*
-		 * On most platforms, we leave the socket open until the process dies.
-		 * This allows clients to perform a "synchronous close" if they care
-		 * --- wait till the transport layer reports connection closure, and
-		 * you can be sure the backend has exited.  Saves a kernel call, too.
+		 * Formerly we did an explicit close() here, but it seems better to
+		 * leave the socket open until the process dies.  This allows clients
+		 * to perform a "synchronous close" if they care --- wait till the
+		 * transport layer reports connection closure, and you can be sure the
+		 * backend has exited.
 		 *
-		 * However, that does not work on Windows: if the kernel closes the
-		 * socket it will invoke an "abortive shutdown" that discards any data
-		 * not yet sent to the client.  (This is a flat-out violation of the
-		 * TCP RFCs, but count on Microsoft not to care about that.)  To get
-		 * the spec-compliant "graceful shutdown" behavior, we must invoke
-		 * closesocket() explicitly.  When using OpenSSL, it seems that clean
-		 * shutdown also requires an explicit shutdown() call.
-		 *
-		 * This code runs late enough during process shutdown that we should
-		 * have finished all externally-visible shutdown activities, so that
-		 * in principle it's good enough to act as a synchronous close on
-		 * Windows too.  But it's a lot more fragile than the other way.
+		 * We do set sock to PGINVALID_SOCKET to prevent any further I/O,
+		 * though.
 		 */
-#ifdef WIN32
-		shutdown(MyProcPort->sock, SD_SEND);
-		closesocket(MyProcPort->sock);
-#endif
-
-		/* In any case, set sock to PGINVALID_SOCKET to prevent further I/O */
 		MyProcPort->sock = PGINVALID_SOCKET;
 	}
 }