Remove useless assignment from tui_remove_hooks
    
    tui_remove_hooks clears deprecated_query_hook, but nothing in the TUI
    ever sets it; so remove the assignment.
    
    gdb/ChangeLog
    2019-08-16  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-hooks.c (tui_remove_hooks): Don't set
    	deprecated_query_hook.