Fix decompose_http_proxy_url

this morning: More Sygnal#130 (HTTP proxy) rework, I'm feeling it
straightening out a lot so hopefully it'll be back in the queue soon;

today: 'More of that'; Catch up on #2528 and #7828 which Riiich has
suggested solving first
Github
matrix-org/synapse#2528 : Homeservers don't
catch up with missed traffic until someone sends another event
matrix-org/synapse#7828 : in-memory federation
transaction transmission queues build up indefinitely for offline
servers

sygnal/helper/proxy/__init__.py

@@ -1,7 +1,7 @@
 from urllib.parse import urlparse
 
 
-def decompose_http_proxy_url(self, proxy_url):
+def decompose_http_proxy_url(proxy_url):
     url = urlparse(proxy_url, scheme="http")
 
     if not url.hostname:
@@ -14,3 +14,5 @@ def decompose_http_proxy_url(self, proxy_url):
             f"Unknown proxy scheme {url.scheme};" f" only 'http' is supported."
         )
 
+    return url
+