staging: goldfish: remove GPL boiler plate text

The SPDX identifier is a legally binding shorthand,
which can be used instead of the full boiler plate text.

There is a SPDX identifier line in goldfish_audio.c.
Remove the GPL boiler plate text to reduce file size.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>