ata: libata-core: Remove unused function

Remove the function ata_do_simple_cmd() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Tejun Heo <tj@kernel.org>