ACPI: OSL: Clean up the removal of unused memory mappings

Fold acpi_os_map_cleanup_deferred() into acpi_os_map_remove() and
pass the latter to INIT_RCU_WORK() in acpi_os_drop_map_ref() to make
the code more straightforward.

No intentional functional impact.

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