Clean up tui_layout_command

Clean up tui_layout_command

tui_layout_command is a simple wrapper for tui_set_layout_by_name.
This removes the extra layer and cleans up the resulting function a
bit -- changing it to call error rather than return a result.  This
necessitated a small change to tui-regs.c, to avoid calling the
function that is being removed.

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

	* tui/tui.h (tui_set_layout_by_name): Don't declare.
	* tui/tui-regs.c (tui_reg_layout): New function.
	(tui_show_registers, tui_reg_command): Use it.
	* tui/tui-layout.c (LAYOUT_USAGE): Remove.
	(tui_layout_command): Rename from tui_set_layout_by_name.  Change
	parameters.
	(tui_layout_command): Remove.