ACPI: bus: Drop acpi_get_first_physical_node()

Fold acpi_get_first_physical_node() into acpi_primary_dev_companion()
that is its only remaining caller and drop its signature from the
header file.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Link: https://patch.msgid.link/9732391.rMLUfLXkoz@rafael.j.wysocki