platform / thinkpad: Remove deprecated hotkey_report_mode parameter

It is somewhat strange that the default value to support the
depracated interface is set (1).

Anyway this has existed for years.  The previous patch already
removed the functionality to still export events through /proc.
Now this param is useless and should vanish too.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Matthew Garrett <matthew.garrett@nebula.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>