manual: remove 'Events' section

The list of buildroot events is outdated, a sign that the manual is not the
best place to refer to timely things like events. The wiki page is better
suited for such information.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>