Use error_no_arg in TUI

Use error_no_arg in TUI

This changes a couple of TUI commands to use error_no_arg.  The
commands are also simplified a bit, and changed to use other gdb CLI
utility functions like skip_to_space.  This lets us removes a couple
of defines that don't interact properly with gettext.

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

	* tui/tui-win.c (tui_set_focus_command)
	(tui_set_win_height_command): Use error_no_arg.
	(_initialize_tui_win): Update help text.
	(FOCUS_USAGE, WIN_HEIGHT_USAGE): Don't define.

Change-Id: I2bf95c2e5cfe1472d068388fa39f0cf07591b76c