Assume that we have functional, 64-bit fseeko()/ftello().

Windows has this, and so do all other live platforms according to the
buildfarm, so remove the configure probe and src/port/ substitution.

Keep the probe that detects whether _LARGEFILE_SOURCE has to be
defined to get that, though ... that seems to be still relevant in
some places.

This is part of a series of commits to get rid of no-longer-relevant
configure checks and dead src/port/ code.  I'm committing them separately
to make it easier to back out individual changes if they prove less
portable than I expect.

Discussion: https://postgr.es/m/15379.1582221614@sss.pgh.pa.us

src/include/port.h

@@ -331,13 +331,9 @@ extern int	gettimeofday(struct timeval *tp, struct timezone *tzp);
  * When necessary, these routines are provided by files in src/port/.
  */
 
-/* WIN32 handled in port/win32_port.h */
-#ifndef WIN32
+/* Type to use with fseeko/ftello */
+#ifndef WIN32					/* WIN32 is handled in port/win32_port.h */
 #define pgoff_t off_t
-#ifdef __NetBSD__
-extern int	fseeko(FILE *stream, off_t offset, int whence);
-extern off_t ftello(FILE *stream);
-#endif
 #endif
 
 extern double pg_erand48(unsigned short xseed[3]);
@@ -349,11 +345,6 @@ extern void pg_srand48(long seed);
 extern int	fls(int mask);
 #endif
 
-#ifndef HAVE_FSEEKO
-#define fseeko(a, b, c) fseek(a, b, c)
-#define ftello(a)		ftell(a)
-#endif
-
 #ifndef HAVE_GETPEEREID
 /* On Windows, Perl might have incompatible definitions of uid_t and gid_t. */
 #ifndef PLPERL_HAVE_UID_GID