scripts/kallsyms: remove last remnants of --page-offset option

The implementation of the --page-offset kallsyms command line option has
been removed, so remove it from the usage string as well.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.com>