Remove configure probe for link.

link() is in SUSv2 and all targeted Unix systems have it.  We have
replacement code for Windows that doesn't require a configure probe.
Since only Windows needs it, rename src/port/link.c to win32link.c like
other similar things.

There is no need for a vestigial HAVE_LINK macro, because we expect all
Unix and, with our replacement function, Windows systems to have it, so
we didn't have any tests around link() usage.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/port/win32link.c

@@ -1,21 +1,19 @@
 /*-------------------------------------------------------------------------
  *
- * link.c
+ * win32link.c
  *
  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  *
  * IDENTIFICATION
- *	  src/port/link.c
+ *	  src/port/win32link.c
  *
  *-------------------------------------------------------------------------
  */
 
 #include "c.h"
 
-#ifdef WIN32
-
 int
 link(const char *src, const char *dst)
 {
@@ -31,5 +29,3 @@ link(const char *src, const char *dst)
 	else
 		return 0;
 }
-
-#endif