mainboard/[g-p]*: Remove copyright notices

mainboard/[g-p]*: 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: I426518e8e18de1c8efcfb7ecb0835df3e257dca1
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39608
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>