Revert "monitoring: add config class"

This reverts commit 6a79118623d15560ca58ecb1fac6ea2a41346a63.

The
 Class['monitoring::configuration'] -> Monitoring::Host <| |>
 Class['monitoring::configuration'] -> Monitoring::Service <| |>
 Class['monitoring::configuration'] -> Monitoring::Group <| |>
syntax does not just create a dependency but also realizes all exported
resources.

As the class gets included on every host (via base::monitoring::host),
this means that every single host realizes exported resources and writes
Nagios configuration files to /etc/nagios.

Revert for now, until a better solution is found.

Change-Id: I5754ac714ee204e8f07d443ccd94ada5769120a6