doc/man: remove AUTHORS section

Many people have contributed to LTTng-tools and the original authors are
not necessarily the top ones now. For instance, GitHub reports this top
5 as of this patch:

1. jgalar    952 commits  45,889 ++  13,959 --
2. compudj    880 commits  56,579 ++  28,095 --
3. PSRCode    303 commits  25,553 ++  10,909 --
4. eepp    199 commits  12,119 ++  6,461 --
5. jdesfossez    174 commits  32,546 ++  3,381 --

Also, man-pages(7), the conventions for writing Linux man pages, says:

> Use of an AUTHORS section is strongly discouraged.

Let's just keep the project's copyrights and some special thanks without
selecting specific authors to list on each man page.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>