nrpe: do not install nagios-plugins-extra

nagios-plugins-extra is an Ubuntuism; it's a custom patch on top of the
nagios-plugins Debian package that creates it. Moreover, it seems to
only contain two(!?) plugins: check_fping & check_game, neither of which
we use. Drop the superfluous dependency.

Change-Id: I69cdaa3508ddd43fd64c4ae6c8906609a9d91829