fix(disco): clean up autoping warning message handling

Remove unused return value from _disco_autoping_warning_message by
changing its return type to void. Rename local variable from
users_prefers_warning to user_prefers_warning for correct grammar.

Use g_ascii_strcasecmp instead of g_strcmp0 for the domain match
check, so the warning fires correctly regardless of case differences
between the stanza 'from' field and the bound domain.

Remove duplicate autoping warning display from cons_notify_setting(),
keeping it only in cons_autoping_setting() where it belongs.

src/xmpp/iq.c

@@ -2432,8 +2432,7 @@ _disco_info_response_id_handler_onconnect(xmpp_stanza_t* const stanza, void* con
         }
 
         // Prevent repetitions by avoiding checks of disco items (from connection_set_disco_items)
-        const char* domain = connection_get_domain();
-        if (from && domain && g_ascii_strcasecmp(from, domain) == 0) {
+        if (from && g_ascii_strcasecmp(from, connection_get_domain()) == 0) {
             _disco_autoping_warning_message(features);
         }
     }