Removing the server_idle_suicide_seconds option

This option existed so that the user can tweak it if they found the default idle
timeout too short, for instance if they leave their machine on over the weekend.

This use case is now covered by the new YcmdKeepalive system that pings ycmd
every 10 minutes as long as Vim is running. This prevents ycmd shutting down if
one leaves their Vim instance alone for a long time.

Thus the old option is useless now; ycmd now shuts down after 3 hours of
inactivity, which should only ever happen when its corresponding Vim instance
has shut down abnormally.