ata: libata: Remove obsolete function declarations

The function ata_schedule_scsi_eh() was removed with commit
f8bbfc24 ("[PATCH] SCSI: make scsi_implement_eh() generic API for
SCSI transports"), and the function ata_sff_irq_clear() was removed
with commit 37f65b8b("libata-sff: ata_sff_irq_clear() is BMDMA
specific").

Remove the now useless declarations of these functions in
drivers/ata/libata.h and include/linux/libata.h.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>