Remove make_command_window

Remove make_command_window

This unifies the creation and re-initialization cases for the command
window.  When this is done, it becomes clear that make_command_window
isn't needed -- it can be replaced with a simple "new", so this is
removed as well.

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

	* tui/tui-layout.c (make_command_window): Remove.
	(show_source_disasm_command, show_source_or_disasm_and_command):
	Unify creation and re-initialization cases.