thinkpad-acpi: remove HKEY disable functionality

thinkpad-acpi: remove HKEY disable functionality

The HKEY disable functionality basically cripples the entire event
model of the ThinkPad firmware and of the thinkpad-acpi driver.
Remove this functionality from the driver.  HKEY must be enabled
at all times while thinkpad-acpi is loaded, and disabled otherwise.

For sysfs, according to the sysfs ABI and the thinkpad-acpi sysfs
rules of engagement, we just remove the attributes.

For procfs, we call WARN() for now.  The leftover cruft on procfs will
be removed in a few months if the warning doesn't trigger.

I don't expect any fallout from this, there really isn't any reason
to mess with hotkey_enable or with the enable/disable commands to
/proc/acpi/ibm/hotkey, and this has been true for _years_.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>