thinkpad-acpi: drop ibm-acpi alias

The driver was renamed two years ago, on 2.6.21.  Drop the old
compatibility alias, we have given everybody quite enough time
to update their configs to the new name.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>