Remove dead pread and pwrite replacement code.

pread() and pwrite() are in SUSv2, and all targeted Unix systems have
them.

Previously, we defined pg_pread and pg_pwrite to emulate these function
with lseek() on old Unixen.  The names with a pg_ prefix were a reminder
of a portability hazard: they might change the current file position.
That hazard is gone, so we can drop the prefixes.

Since the remaining replacement code is Windows-only, move it into
src/port/win32p{read,write}.c, and move the declarations into
src/include/port/win32_port.h.

No need for vestigial HAVE_PREAD, HAVE_PWRITE macros as they were only
used for declarations in port.h which have now moved into win32_port.h.

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

src/include/port/win32_port.h

@@ -562,4 +562,10 @@ typedef unsigned short mode_t;
 #define HAVE_BUGGY_STRTOF 1
 #endif
 
+/* in port/win32pread.c */
+extern ssize_t pread(int fd, void *buf, size_t nbyte, off_t offset);
+
+/* in port/win32pwrite.c */
+extern ssize_t pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
+
 #endif							/* PG_WIN32_PORT_H */