ata: remove unused helper ata_id_flush_enabled()

Not only is this function unused, but even worse, the bit it is checking
is actually used for signaling if the feature is supported, not enabled.
Therefore, remove the unused helper function ata_id_flush_enabled().

ata_id_has_flush() is left unmodified, since this extra supported bit
(Bit 12 of word 86) is simply a copy of the bit that ata_id_has_flush()
already checks (Bit 12 of word 83), see ACS-5 r10:
7.13.6.41 Words 85..87, 120: Commands and feature sets supported or enabled

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>