Removes redundant variable assignments for Debian

Puppet 3.1.1 is reporting the following errors during a vagrant run:
  - Error: Cannot reassign variable id_dir at
    /tmp/vagrant-puppet/modules-0/monit/manifests/params.pp:50
    on node debian-70rc1-x64-vbox4210.vagrantup.com
  - Error: Cannot reassign variable id_dir at
    /tmp/vagrant-puppet/modules-0/monit/manifests/params.pp:50
    on node debian-70rc1-x64-vbox4210.vagrantup.com