kasan: drop unnecessary GPL text from comment headers

Don't mention "GNU General Public License version 2" text explicitly,
as it's already covered by the SPDX-License-Identifier.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Marco Elver <elver@google.com>
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Change-Id: If0a2690042a2aa0fca70cea601ae9aabe72fa233