timegm.3: Remove recommendation against their use

timegm.3: Remove recommendation against their use

It was straight after a note that they are nonstandard functions,
which already tells the user that if portability is in mind, they
shouldn't be used, so this recommendation adds nothing in that
sense.

Also, there's a note that timelocal() should _never_ be used, due
to mktime() being identical and in the POSIX standard (it is also
in C99), so this note would also add nothing in that sense.

So the only uses not covered by those other notes are non-portable
uses of timegm(3).  In that scenario, it is an excellent function.

Cc: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>