block: ahci: Remove dead code

CONFIG_AHCI_SETFEATURES_XFER is not selected by any user, so delete
the dead code.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>