Remove "data_content" and "data_content_count" from TUI data window

Remove "data_content" and "data_content_count" from TUI data window

The TUI has some stub code for adding data other than registers to the
data window.  However, it doesn't do anything, and apparently never
has.  This removes the dead code.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.c (tui_data_item_window): Update.
	* tui/tui-windata.h (tui_check_data_values): Don't declare.
	* tui/tui-windata.c (tui_display_all_data)
	(tui_display_data_from_line): Update.
	(tui_check_data_values): Remove.
	* tui/tui-regs.c (tui_show_register_group)
	(tui_display_reg_element_at_line): Update.
	* tui/tui-hooks.c (tui_register_changed)
	(tui_refresh_frame_and_register_information): Call
	tui_check_register_values.
	* tui/tui-data.h (struct tui_data_window) <data_content,
	data_content_count, data_type>: Remove.
	(enum tui_data_type): Remove.