Remove default pool, it's used automatically anyway.

lib/pleroma/http/connection.ex

@@ -8,7 +8,6 @@ defmodule Pleroma.HTTP.Connection do
   """
 
   @hackney_options [
-    pool: :default,
     timeout: 10000,
     recv_timeout: 20000,
     follow_redirect: true

lib/pleroma/http/http.ex

@@ -56,7 +56,6 @@ defp process_sni_options(options, url) do
   def process_request_options(options) do
     config = Application.get_env(:pleroma, :http, [])
     proxy = Keyword.get(config, :proxy_url, nil)
-    options = options ++ [adapter: [pool: :default]]
 
     case proxy do
       nil -> options

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -741,8 +741,7 @@ def publish_one(%{inbox: inbox, json: json, actor: actor, id: id}) do
         {"Content-Type", "application/activity+json"},
         {"signature", signature},
         {"digest", digest}
-      ],
-      hackney: [pool: :default]
+      ]
     )
   end