Change tui_set_layout to return void

Change tui_set_layout to return void

tui_set_layout can't meaningfully be called with UNDEFINED_LAYOUT; and
instead of trying to handle this case, simply assert and have the
function return void.  No caller was checking the return value anyway.

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

	* tui/tui-layout.h (tui/tui-layout): Return void.
	* tui/tui-layout.c (tui_set_layout): Return void.  Add assert.