MAINTAINERS: Remove Bjorn Helgaas as PNP maintainer

I don't have time or knowledge to add anything useful to PNP, so remove
myself from the PNP entry.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>