MRF: fix HashtagPolicy as unconfigurable default

#2651 (closed)

Took me a sec to figure out what was going on here, but I think a piece from development of !3223 (merged) got accidentally committed as a mistake:

  def get_policies do
    Pleroma.Config.get([:mrf, :policies], [])
    |> get_policies()
    |> Enum.concat([Pleroma.Web.ActivityPub.MRF.HashtagPolicy])
  end

This effectively makes HashtagPolicy a ghost default MRF with no way to see it or alter it in the config.

Removing this gets rid of a compile-time cycle and shrinks many others: https://www.diffchecker.com/qxdINZXe

@lanodan please review.