ata: libata-acpi: kill ata_acpi_on_suspend()

Since the commit c05e6ff0 ("libata-acpi: implement
and use ata_acpi_init_gtm()") ata_acpi_on_suspend() just returns 0, so
its call from ata_eh_handle_port_suspend() doesn't make sense anymore.
Remove the function completely, at last...

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>