Remove mention of openproject-plugins in README

Signed-off-by: Alex Coles <alex@alexbcoles.com>

README.md

@@ -6,7 +6,6 @@ Adds support for OmniAuth OpenID Connect strategy providers, most importantly Go
 
 You will have to add the following lines to your OpenProject's _Gemfile.plugins_ for the time being:
 
-    gem "openproject-plugins", :git => "git@github.com:opf/openproject-plugins.git", :branch => "dev"
     gem "openproject-auth_plugins", :git => 'git@github.com:finnlabs/openproject-auth_plugins', :branch => 'dev'
     gem 'omniauth-openid-connect', :git => 'git@github.com:finnlabs/omniauth-openid-connect.git', :branch => 'master'
     gem 'openproject-openid_connect', :git => 'git@github.com:finnlabs/openproject-openid_connect.git', :branch => 'dev'