river: remove dead code

This file has been dead since the refactor to use the wlroots
scene graph deleted view_stack.zig

river/test_main.zig

@@ -1,19 +0,0 @@
-// This file is part of river, a dynamic tiling wayland compositor.
-//
-// Copyright 2020 The River Developers
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, version 3.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with this program. If not, see <https://www.gnu.org/licenses/>.
-
-test "river test suite" {
-    _ = @import("view_stack.zig");
-}