[test] The debugging proxy cucumber step is not available anymore

Go's official proxy implementation doesn't allow any proxy configuration
from environment variables to be applied to requests to `localhost`.

features/steps.rb

@@ -157,12 +157,6 @@ Given(/^the GitHub API server:$/) do |endpoints_str|
   set_environment_variable 'HUB_TEST_HOST', "http://127.0.0.1:#{@server.port}"
 end
 
-Given(/^I use a debugging proxy(?: at "(.+?)")?$/) do |address|
-  address ||= 'localhost:8888'
-  set_environment_variable 'HTTP_PROXY', address
-  set_environment_variable 'HTTPS_PROXY', address
-end
-
 Then(/^shell$/) do
   cd('.') do
     system '/bin/bash -i'