libata: remove ATA_FLAG_LPM

Commit 6b7ae954 (libata: reimplement link power
management) removed the check of ATA_FLAG_LPM but neglected to remove the flag
itself.  Do it now...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>