Remove FIXMEs from tui-layout.c

Remove FIXMEs from tui-layout.c

An earlier patch added a couple of FIXME comments to tui-layout.c.
This removes them.  This is possible due to the previous patch that
changed how boxing works in the TUI -- now, no special case for the
command window is needed in box_win.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_source_disasm_command)
	(show_source_or_disasm_and_command): Use make_visible method, not
	tui_make_window.
	* tui/tui-command.h (struct tui_cmd_window) <make_visible>:
	Remove.