luci-mod-admin-full: remove unused button cbi model

Remove the unused admin_system/buttons cbi model. It has been broken,
was not exposed in the controller and the corresponding support code
which only ever was available for the ath25 target has been dropped in
with https://git.lede-project.org/0a3d721in June 2016.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>