libata: acpi: remove dead code for ata_acpi_(un)bind

Commit 7381fe73 "libata-acpi: remove redundent code for power resource
handling" removed ata_acpi_(un)bind but left their prototypes in libata.h,
so remove them.

Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>