staging: pi433: remove GPL boiler plate text

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

All source and header files have SPDX identifiers.
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>