Remove mail_google_schema_whitelisting spec

This task was removed in 2014! The spec wasn't, probably because it
didn't fail as it was never executed. See
0fc5c802.

spec/tasks/gitlab/mail_google_schema_whitelisting_spec.rb

-require 'spec_helper'
-require 'rake'
-
-describe 'gitlab:mail_google_schema_whitelisting rake task' do
-  before :all do
-    Rake.application.rake_require "tasks/gitlab/helpers"
-    Rake.application.rake_require "tasks/gitlab/mail_google_schema_whitelisting"
-    # empty task as env is already loaded
-    Rake::Task.define_task :environment
-  end
-
-  describe 'call' do
-    before do
-      # avoid writing task output to spec progress
-      allow($stdout).to receive :write
-    end
-
-    let :run_rake_task do
-      Rake::Task["gitlab:mail_google_schema_whitelisting"].reenable
-      Rake.application.invoke_task "gitlab:mail_google_schema_whitelisting"
-    end
-
-    it 'runs the task without errors' do
-      expect { run_rake_task }.not_to raise_error
-    end
-  end
-end