SS/USSD: release IUSE sessions immediately after response

At the moment, all available IUSE handlers do assume a single
request-response operation, e.g. MS requests its MSISDN - IUSE
responds. No further nor intermediate communications is required.

Let's immediately terminate such SS sessions in order to avoid
waiting for the session inactivity watchdog (i.e. timeout).

Change-Id: Iaefe37512da79e10fbe92378236bfff0eae0f8b9
diff --git a/src/hlr_ussd.c b/src/hlr_ussd.c
index ba373f3..b7aaba4 100644
--- a/src/hlr_ussd.c
+++ b/src/hlr_ussd.c
@@ -455,6 +455,8 @@
 		} else {
 			/* Handle internally */
 			ss->u.iuse->handle_ussd(conn, ss, gsup, req);
+			/* Release session immediately */
+			ss_session_free(ss);
 		}
 	}