Remove tui_scroll_direction enum

Remove tui_scroll_direction enum

The tui_scroll_direction enum is not really needed, because it's
simple to adapt the various scrolling methods to use the sign of their
argument as the direction in which to scroll.

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

	* tui/tui-winsource.c
	(tui_source_window_base::do_scroll_horizontal): Remove direction
	parameter.
	* tui/tui-windata.c (tui_data_window::do_scroll_vertical): Remove
	direction parameter.
	* tui/tui-win.c (tui_win_info::forward_scroll)
	(tui_win_info::backward_scroll, tui_win_info::left_scroll)
	(tui_win_info::right_scroll): Update.
	* tui/tui-source.c (tui_source_window::do_scroll_vertical): Remove
	direction parameter.
	* tui/tui-disasm.c (tui_disasm_window::do_scroll_vertical): Remove
	direction parameter.
	* tui/tui-data.h (enum tui_scroll_direction): Remove.
	(struct tui_win_info) <do_scroll_vertical, do_scroll_horizontal>:
	Remove direction parameter.
	(struct tui_source_window_base, struct tui_source_window)
	(struct tui_disasm_window, struct tui_data_window)
	(struct tui_cmd_window): Update.