Remove start_screen() and end_screen() from the
Window-port interface.
They were only ever used by tty, and there was a comment
carried to several window-ports about how they "really
should go away. They are tty-specific"
term_start_screen() and term_end_screen() are part of
terminal/NO_TERMS supporting routines now.
If an old port is resurrected to work with current version code, its files
can be relocated to the appropriate sys or win folder as required.
In the meantime, the burden of upkeep can be avoided for the stuff in the
outdated folder for now.
