Fix lchan refcounting in case of IMSI DETACH

Removed lchan_put() after IMSI detach function.
We don't need to put lchan, because we don't hold a ressource.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index d5f12de..05af593 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -1505,8 +1505,6 @@
 	} else
 		DEBUGP(DMM, "Unknown Subscriber ?!?\n");
 
-	put_lchan(msg->lchan);
-
 	return 0;
 }