EVENTHANDLER(9): drop the list of event handlers

Maintaining a comprehensive list of event handlers in this man page is a
futile endeavor. It is entirely detached from the source code, and
therefore requires that anyone adding/removing an event handler have
prior knowledge of the list. Many do not, so it will naturally become
stale (and is).

This is demonstrated by the fact that there are currently 88 instances
of EVENTHANDLER_DECLARE() in the source tree, but the list contains 66
items.

Many of the descriptions do not offer much detail that could not be
gleaned from the handler name alone. It is a more effective strategy to
document the purpose/details of the event handler in a comment alongside
its declaration.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36895