CLEANUP: session: remove session_from_task()

Since commit 3dd6a25 ("MINOR: stream-int: retrieve session pointer from
stream-int"), we can get the session from the task, so let's get rid of
this less obvious function.
diff --git a/src/peers.c b/src/peers.c
index 173f784..23c4ea3 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -178,7 +178,7 @@
  */
 static void peer_session_release(struct stream_interface *si)
 {
-	struct session *s = session_from_task(si->owner);
+	struct session *s = si_sess(si);
 	struct appctx *appctx = objt_appctx(si->end);
 	struct peer_session *ps = (struct peer_session *)appctx->ctx.peers.ptr;
 
@@ -212,7 +212,7 @@
  */
 static void peer_io_handler(struct stream_interface *si)
 {
-	struct session *s = session_from_task(si->owner);
+	struct session *s = si_sess(si);
 	struct peers *curpeers = (struct peers *)s->fe->parent;
 	struct appctx *appctx = objt_appctx(si->end);
 	int reql = 0;