doc: remove redundant copyright page

Copyright page does not contain any useful or updated information. Every
Sphinx page is already copyrighted with the message:

> (c) Copyright 2015-2022 Zephyr Project members and individual
  contributors.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/copyright.rst

@@ -1,9 +0,0 @@
-:orphan:
-
-.. _doc-copyrights:
-
-Documentation Copyrights
-########################
-
-* 2015-2017 Intel Corporation
-* 2015-2017 Wind River Systems, Inc