CLEANUP: vars: remove unused struct

The commit "MEDIUM: vars: move the session variables to the session, not the stream" (ebcd4844e82a4198ea5d98fe491a46267da1d1ec")
moves the variables from the stream to the session. It forgot to remove
the stream definition of the "vars_sess".
diff --git a/include/types/stream.h b/include/types/stream.h
index 953f9d1..bba5f43 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -144,7 +144,6 @@
 
 	char **req_cap;                         /* array of captures from the request (may be NULL) */
 	char **res_cap;                         /* array of captures from the response (may be NULL) */
-	struct vars vars_sess;                  /* list of variables for the session scope. */
 	struct vars vars_txn;                   /* list of variables for the txn scope. */
 	struct vars vars_reqres;                /* list of variables for the request and resp scope. */