Revert "gdbus: Use DBUS_SESSION_BUS_ADDRESS if AT_SECURE but not setuid"

This reverts commit 7aa0580cc5.

As stated in #2316, that commit was a workaround to allow gnome-keyring
and msmtp to continue to get their session bus address from
`DBUS_SESSION_BUS_ADDRESS`, even though they’re `AT_SECURE`. The timeout
on that workaround has expired so that commit is now being reverted.

Fixes: #2316

gio/gdbusaddress.c

@@ -1343,31 +1343,9 @@ g_dbus_address_get_for_bus_sync (GBusType       bus_type,
 
     case G_BUS_TYPE_SESSION:
       if (has_elevated_privileges)
-        {
+        ret = NULL;
-#ifdef G_OS_UNIX
-          if (geteuid () == getuid ())
-            {
-              /* Ideally we shouldn't do this, because setgid and
-               * filesystem capabilities are also elevated privileges
-               * with which we should not be trusting environment variables
-               * from the caller. Unfortunately, there are programs with
-               * elevated privileges that rely on the session bus being
-               * available. We already prevent the really dangerous
-               * transports like autolaunch: and unixexec: when our
-               * privileges are elevated, so this can only make us connect
-               * to the wrong AF_UNIX or TCP socket. */
-              ret = g_strdup (g_getenv ("DBUS_SESSION_BUS_ADDRESS"));
-            }
-          else
-#endif
-            {
-              ret = NULL;
-            }
-        }
       else
-        {
+        ret = g_strdup (g_getenv ("DBUS_SESSION_BUS_ADDRESS"));
-          ret = g_strdup (g_getenv ("DBUS_SESSION_BUS_ADDRESS"));
-        }
 
       if (ret == NULL)
         {