mm_state_iu_fsm: T3314 expiry must lead to PMM IDLE, not PMM DETACHED

This Iu timer is Osmocom specific, but is made to resemble T3314 timer
from GERAN (also named READY timer). The READY timer mission is to make
the MM state transition from READY to STANDBY, which in PMM (UTRAN)
matches the transition from CONNECTED to IDLE.
Instead, the patch introducing the timer was making it transition to
DETACHED directly, but this was clearly not the intention:
* Detaching a UE after 44 seconds (default value for T3314) is overkill.
* The comment describing it says: "Iu User inactivity timer. On expiry
  release Iu connection". The release of Iu connection happens during
  the CONNECTED->IDLE transition (that's basically the difference between
  both states).

The transition CONNECTED->IDLE is done by means of calling
sgsn_ranap_iu_release_free(), which will eventually answer with a event
RANAP_IU_EVENT_IU_RELEASE from lower layers when the Release Complete is
received. At that point, osmo-sgsn code frees the connection and
transitions to IDLE state. This way we maintain the state according to
the connection existance.

Related: SYS#5389
Related: osmo-iuh.git Change-Id Iac822c74e56750dc40e94573eae0e20853ff68c0
Fixes: 3bad31bcb4
Change-Id: I7279102ad51b0c39eb6d04c129986984112d15cc

src/sgsn/gprs_mm_state_iu_fsm.c

@@ -64,7 +64,6 @@ static void st_pmm_connected(struct osmo_fsm_inst *fi, uint32_t event, void *dat
 		break;
 	case E_PMM_USER_INACTIVITY:
 		sgsn_ranap_iu_release_free(ctx, &user_inactive_cause);
-		mm_state_iu_fsm_state_chg(fi, ST_PMM_DETACHED);
 		break;
 	case E_PMM_RA_UPDATE:
 		break;