mainboard/google: Remove copyright notices

mainboard/google: 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: I09cc279b1f75952bb397de2c3f2b299255163685
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39607
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>