Remove two unused functions from the TUI

This removes two unused functions from the TUI.  According to
"git grep -G", they have never been used.

gdb/ChangeLog
2019-06-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (tui_del_window, tui_del_data_windows): Don't
	declare.
	* tui/tui-data.c (tui_del_window, tui_del_data_windows): Remove.
3 files changed