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.