platform/x86: thinkpad_acpi: Drop ACPI driver registration

There is no point in registering an ACPI driver that only has an empty
.add() callback, which is done by the thinkpad_acpi driver, since
after binding to an ACPI device it only sits there and does nothing.

That binding only effectively causes the ACPI device's reference count
to increase, but that can be achieved by using acpi_get_acpi_dev()
instead of acpi_fetch_acpi_dev() in setup_acpi_notify(), and doing
the corresponding cleanup in ibm_exit().

Update the code accordingly and get rid of the non-functional ACPI
driver.

No intentional functional impact beyond altering sysfs content.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Link: https://patch.msgid.link/3949487.kQq0lBPeGt@rafael.j.wysocki