mainboard/[a-f]*: Remove copyright notices

mainboard/[a-f]*: 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: I57fc98788bb47df16d6aedd0f0701e9991801743
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39606
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by: Tristan Corrick <tristan@corrick.kiwi>