gdb/tui: restore delete of window objects

gdb/tui: restore delete of window objects

In commit:

  commit f237f998d1168139d599c550d54169cd8f94052d
  Date:   Mon Jan 25 18:43:19 2021 +0000

      gdb/tui: remove special handling of locator/status window

I accidentally remove a call to delete the tui window objects.  Now
every time GDB changes tui layout it is leaking windows.

gdb/ChangeLog:

	* tui/tui-layout.c (tui_apply_current_layout): Restore the delete
	of the window objects.