serial: 8250: Remove low_latency workaround

The defunct low_latency input steering executed flush_to_ldisc()
directly from interrupt context so dropping the port lock was
necessary to avoid deadlock. That steering was removed by

    commit a9c3f68f
    Author: Peter Hurley <peter@hurleysoftware.com>
    Date:   Sat Feb 22 07:31:21 2014 -0500

    tty: Fix low_latency BUG

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>