http signature plug: remove redundant checks handled by HTTPSignatures library

the redundant checks assumed a POST request, which will not work for signed GETs.
this check was originally needed because the HTTPSignatures adapter assumed that
the requests were also POST requests.  but now, the adapter has been corrected.

lib/pleroma/plugs/http_signature.ex

@@ -3,7 +3,6 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 
 defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
-  alias Pleroma.Web.ActivityPub.Utils
   import Plug.Conn
   require Logger
 
@@ -16,12 +15,9 @@ def call(%{assigns: %{valid_signature: true}} = conn, _opts) do
   end
 
   def call(conn, _opts) do
-    user = Utils.get_ap_id(conn.params["actor"])
-    Logger.debug("Checking sig for #{user}")
     [signature | _] = get_req_header(conn, "signature")
 
-    cond do
+    if signature do
-      signature && String.contains?(signature, user) ->
       # set (request-target) header to the appropriate value
       # we also replace the digest header with the one we computed
       conn =
@@ -40,12 +36,7 @@ def call(conn, _opts) do
         end
 
       assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
-
+    else
-      signature ->
-        Logger.debug("Signature not from actor")
-        assign(conn, :valid_signature, false)
-
-      true ->
       Logger.debug("No signature header!")
       conn
     end

test/plugs/http_signature_plug_test.exs

@@ -26,22 +26,4 @@ test "it call HTTPSignatures to check validity if the actor sighed it" do
       assert called(HTTPSignatures.validate_conn(:_))
     end
   end
-
-  test "bails out early if the signature isn't by the activity actor" do
-    params = %{"actor" => "https://mst3k.interlinked.me/users/luciferMysticus"}
-    conn = build_conn(:get, "/doesntmattter", params)
-
-    with_mock HTTPSignatures, validate_conn: fn _ -> false end do
-      conn =
-        conn
-        |> put_req_header(
-          "signature",
-          "keyId=\"http://mastodon.example.org/users/admin#main-key"
-        )
-        |> HTTPSignaturePlug.call(%{})
-
-      assert conn.assigns.valid_signature == false
-      refute called(HTTPSignatures.validate_conn(:_))
-    end
-  end
 end