soc: Remove copyright notices

They're listed in AUTHORS and often incorrect anyway, for example:
- What's a "Copyright $year-present"?
- Which incarnation of Google (Inc, LLC, ...) is the current
  copyright holder?
- People sometimes have their editor auto-add themselves to files even
  though they only deleted stuff
- Or they let the editor automatically update the copyright year,
  because why not?
- Who is the copyright holder "The coreboot project Authors"?
- Or "Generated Code"?

Sidestep all these issues by simply not putting these notices in
individual files, let's list all copyright holders in AUTHORS instead
and use the git history to deal with the rest.

Change-Id: I4c110f60b764c97fab2a29f6f04680196f156da5
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39610
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>

src/soc/intel/skylake/systemagent.c

@@ -1,9 +1,6 @@
 /*
  * This file is part of the coreboot project.
  *
- * Copyright (C) 2007-2009 coresystems GmbH
- * Copyright (C) 2014 Google Inc.
- * Copyright (C) 2015-2017 Intel Corporation.
  *
  * 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