MINOR: stream: Always get idle duration from the session

Since the idle duration provided by the session is always up-to-date, there
is no more reason to rely on the multiplexer cs_info to set it to the
stream.
diff --git a/src/http_ana.c b/src/http_ana.c
index d89b7d5..6659f7e 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -109,13 +109,8 @@
 	s->srv_error = http_return_srv_error;
 
 	/* If there is data available for analysis, log the end of the idle time. */
-	if (c_data(req) && s->logs.t_idle == -1) {
-		const struct cs_info *csinfo = si_get_cs_info(objt_cs(s->si[0].end));
-
-		s->logs.t_idle = ((csinfo)
-				  ? csinfo->t_idle
-				  : tv_ms_elapsed(&s->logs.tv_accept, &now) - s->logs.t_handshake);
-	}
+	if (c_data(req) && s->logs.t_idle == -1)
+		s->logs.t_idle = sess->t_idle;
 
 	/*
 	 * Now we quickly check if we have found a full valid request.