mxser: remove MOXA_GETDATACOUNT ioctl

1) MOXA_GETDATACOUNT is broken at least since commit ecbbfd44a08f (TTY:
   move tty buffers to tty_port) in 2012.
   Even though it's mentioned in the mon_p moxa utility, obviously noone
   ever tried that in that timeframe.
2) The ioctl definition was never exposed to userspace, so noone should
   actually be reliably using this.

Provided the above, remove this mxser-special ioctl.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-28-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>