pager: remove obsolete comment

The comment at the top of pager.c claims that we've split
the code out so that Windows can do something different.
This dates back to f67b45f (Introduce trivial new pager.c
helper infrastructure, 2006-02-28), because the original
implementation used fork(). Later, we ended up sticking the
Windows #ifdefs into this file anyway. And then even later,
in ea27a18 (spawn pager via run_command interface,
2008-07-22) we unified the implementations.

So these days this comment is really saying nothing at all.
Let's drop it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
