Simplify command window creation

Simplify command window creation

make_command_window is never called when *win_info_ptr is non-NULL, so
this patch simplifies the function by removing the parameter and
having it return its result directly.  This in turn makes it more
obvious that a NULL check in show_source_disasm_command can be
removed.

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

	* tui/tui-layout.c (make_command_window): Remove win_info_ptr
	parameter.  Return the new window.
	(show_source_disasm_command): Update and remove NULL check.
	(show_source_or_disasm_and_command): Update.