Rework UI Auth session validation for registration (#7455)

Be less strict about validation of UI authentication sessions during
registration to match client expecations.

synapse/rest/client/v2_alpha/register.py

@@ -516,6 +516,7 @@ class RegisterRestServlet(RestServlet):
             body,
             self.hs.get_ip_from_request(request),
             "register a new account",
+            validate_clientdict=False,
         )
 
         # Check that we're not trying to register a denied 3pid.