Introduce tui_data_window::delete_data_content_windows method

Introduce tui_data_window::delete_data_content_windows method

This changes tui_delete_data_content_windows to be a method on
tui_data_window, removing some uses of the TUI_DATA_WIN global.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-windata.h (tui_delete_data_content_windows): Don't
	declare.
	* tui/tui-windata.c
	(tui_data_window::delete_data_content_windows): Rename from
	tui_delete_data_content_windows.
	(tui_data_window::display_all_data)
	(tui_data_window::do_scroll_vertical): Update.
	* tui/tui-data.h (struct tui_data_window)
	<delete_data_content_windows>: New method.