TTY: simserial, remove some tty ops

All ->start, ->stop and ->wait_until_sent are empty and need not be
defined. The time to remove them is now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>