Minor cleanups in tui-io.c

Minor cleanups in tui-io.c

I noticed that a couple of functions in tui-io.c could be static, and
that a couple more were unused and could be removed.

gdb/ChangeLog
2018-10-19  Tom Tromey  <tom@tromey.com>

	* tui/tui-io.h (key_is_start_sequence, key_is_end_sequence)
	(key_is_backspace, tui_getc): Don't declare.
	* tui/tui-io.c (key_is_start_sequence): Now static.
	(key_is_end_sequence, key_is_backspace): Remove.
	(tui_getc): Now static.