ARC: Remove unused DT template file

Commit abe11dde "Enabling DeviceTree for Angel4 board" made
angel4.dts as default leaving the orphan skeleton file behind.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/boot/dts/skeleton.dts

-/*
- * Copyright (C) 2012 Synopsys, Inc. (www.synopsys.com)
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- */
-/dts-v1/;
-
-/include/ "skeleton.dtsi"