timegm.3: Remove sample implementation of timegm()

Stephen and Mats both question the wisdom of showing a portable
*non-thread-safe* implementation of timegm(), and I find it
hard to disagree. So, remove this code.

See https://bugzilla.kernel.org/show_bug.cgi?id=103701

Reported-by: Stephen Hurd <shurd@sasktel.net>
Reported-by: Mats Wichmann <mats@linuxfoundation.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/timegm.3

@@ -70,51 +70,13 @@ T}	Thread safety	MT-Safe env locale
 .SH CONFORMING TO
 These functions are nonstandard GNU extensions
 that are also present on the BSDs.
-Avoid their use; see NOTES.
+Avoid their use.
 .SH NOTES
 The
 .BR timelocal ()
 function is equivalent to the POSIX standard function
 .BR mktime (3).
 There is no reason to ever use it.
-.LP
-For a portable version of
-.BR timegm (),
-set the
-.B TZ
-environment variable to UTC, call
-.BR mktime (3)
-and restore the value of
-.BR TZ .
-Something like
-
-.in +4n
-.nf
-#include <time.h>
-#include <stdlib.h>
-
-time_t
-my_timegm(struct tm *tm)
-{
-    time_t ret;
-    char *tz;
-
-    tz = getenv("TZ");
-    if (tz)
-        tz = strdup(tz);
-    setenv("TZ", "", 1);
-    tzset();
-    ret = mktime(tm);
-    if (tz) {
-        setenv("TZ", tz, 1);
-        free(tz);
-    } else
-        unsetenv("TZ");
-    tzset();
-    return ret;
-}
-.fi
-.in
 .SH SEE ALSO
 .BR gmtime (3),
 .BR localtime (3),