snis_client --monitor option has been broken since the SDL2 conversion

[smcameron]

Original commit to support multiple monitors via GTK stuff is here: c31d82b

We still have the code to process the option, but it doesn't do anything but set monitorid, which is never used. (Surprised clang scan-build didn't mark this as a dead store... maybe it did, but I don't remember seeing it.)

$ grep monitorid snis_client.c
static int monitorid = -1;
				monitorid = value;

Some possibly relevant info re multiple monitors and SDL2: https://stackoverflow.com/questions/18728821/multiple-displays-in-sdl2