platform/x86: thinkpad_acpi: Remove unused sensors_pdev_attrs_registered flag

After the recent sysfs-attributes registration cleanups, the
tp_features.sensors_pdev_attrs_registered flag only ever gets set and
never gets read, remove it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20211123210424.266607-6-hdegoede@redhat.com