Remove tui_set_win_with_focus

I noticed that the TUI had two functions with similar names:
tui_set_win_focus_to and tui_set_win_with_focus.

However, one was just an implementation detail of the latter.  So,
this patch removes tui_set_win_with_focus entirely, to avoid any
temptation to call it.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (tui_set_win_focus_to): Move to tui-data.c.
	* tui/tui-data.h (tui_set_win_with_focus): Don't declare.
	* tui/tui-data.c (tui_set_win_with_focus): Remove.
	(tui_set_win_focus_to): Move from tui-win.c.

Change-Id: Idffddab773436bdf80d55480906d76b292981ef2