Remove unused puppet-staging

The puppet-staging module is not currently used by any modules, thus
can be removed from test dependencies.

NOTE:
This backport also covers external_modules.txt, which was addressed
by the follow-up commit[1].

[1] df1b15cb5f
    Remove leftover of puppet-staging

Change-Id: Id9c3f11741b4b0f0117424931daab62db0efa10b
(cherry picked from commit d58599e686)

Puppetfile

@@ -162,10 +162,6 @@ mod 'ipaclient',
   :git => 'https://github.com/joshuabaird/puppet-ipaclient',
   :ref => '2.5.2'
 
-mod 'staging',
-  :git => 'https://github.com/nanliu/puppet-staging',
-  :ref => '1.0.4'
-
 mod 'apache',
   :git => 'https://github.com/puppetlabs/puppetlabs-apache',
   :ref => 'v6.5.1'

external_modules.txt

@@ -2,7 +2,6 @@ antonlindstrom/puppet-powerdns
 camptocamp/puppet-kmod
 duritong/puppet-sysctl
 joshuabaird/puppet-ipaclient
-nanliu/puppet-staging
 puppetlabs/puppetlabs-apache
 puppetlabs/puppetlabs-apt
 puppetlabs/puppetlabs-concat