Remove unused curl_prefix variable
Remove unused curl_prefix variable

With change f223f66894e5e948bcd2cf05d7beaa911605b3ff (Convert to use
'archive' instead of 'staging' for rabbitmqadmin install) the
$curl_prefix variable became unused:
$ grep -ir curl_prefix
manifests/install/rabbitmqadmin.pp:      $curl_prefix = ''
manifests/install/rabbitmqadmin.pp:      $curl_prefix  = "--noproxy ${management_ip_address} -g -6"
manifests/install/rabbitmqadmin.pp:      $curl_prefix  = "--noproxy ${management_ip_address}"

Let's just remove them. Users have means to specify options
to the puppet-archive provider anyways.

Slm0n87/puppet-rabbitmq,Slm0n87/puppet-rabbitmq,voxpupuli/puppet-rabbitmq,voxpupuli/puppet-rabbitmq,Slm0n87/puppet-rabbitmq,puppetlabs/puppetlabs-rabbitmq,puppetlabs/puppetlabs-rabbitmq