tty: simserial: drop unused iflag macro

Drop the RELEVANT_IFLAG() macro which hasn't been used for over a
decade.

Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>