history service: remove unnecessary `global` statement
history service: remove unnecessary `global` statement

The unnecessary global statement leads to a

  SyntaxError: name 'add_headers_cb' is assigned to
  before global declaration

on Python 3.6.

GoelDeepak/dcos,dcos/dcos,kensipe/dcos,GoelDeepak/dcos,GoelDeepak/dcos,amitaekbote/dcos,mellenburg/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,branden/dcos,kensipe/dcos,branden/dcos,surdy/dcos,kensipe/dcos,mellenburg/dcos,surdy/dcos,dcos/dcos,mellenburg/dcos,amitaekbote/dcos,branden/dcos,surdy/dcos,mesosphere-mergebot/mergebot-test-dcos,amitaekbote/dcos,dcos/dcos,mesosphere-mergebot/dcos,amitaekbote/dcos,mesosphere-mergebot/mergebot-test-dcos,mellenburg/dcos,mesosphere-mergebot/mergebot-test-dcos,kensipe/dcos,surdy/dcos,mesosphere-mergebot/dcos,dcos/dcos,dcos/dcos,GoelDeepak/dcos,mesosphere-mergebot/dcos,branden/dcos