staging: gasket: remove TODO item about SPDX usage

Now that the files are all properly tagged with SPDX lines, and the
boilerplate license text is gone, remove the TODO item.

Cc: Rob Springer <rspringer@google.com>
Cc: John Joseph <jnjoseph@google.com>
Cc: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>