libata: acpi: Remove ata_dev_acpi_handle stub in libata.h

The ata_dev_acpi_handle is defined in libata-acpi.c and the only
external user is libata-zpodd.c, which is built when CONFIG_ATA_ACPI
is set, so there is no need to make an empty stub function for
ONFIG_ATA_ACPI case in libata.h. It also causes compile errors due to
acpi_handle is not defined when !CONFIG_ACPI. This patch fixes this
problem by removing the empty stub.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>