Cleanup a stray instance variable

This was added as part of the previous filtering via RSpec::Config.  It
should have been removed previously as part of moving the logic to
specify the tags we wanted in Specs::Runner

qa/qa/specs/config.rb

@@ -13,7 +13,6 @@ module QA
 
       def initialize
         @address = ENV['GITLAB_URL']
-        @exclusion_filter = { mattermost: true }
       end
 
       def perform