[gdb/tui] Fix assert in ~gdbpy_tui_window_maker

[gdb/tui] Fix assert in ~gdbpy_tui_window_maker

In gdb/tui/tui-layout.c, we have:
...
static window_types_map known_window_types;
...
and in gdb/python/py-tui.c:
...
  /* A global list of all gdbpy_tui_window_maker objects.  */
  static intrusive_list<gdbpy_tui_window_maker> m_window_maker_list;
};

/* See comment in class declaration above.  */

intrusive_list<gdbpy_tui_window_maker>
  gdbpy_tui_window_maker::m_window_maker_list;
...

With a gdb build with -O0 or -O2, the static destructor calling order seems to be:
- first gdb/tui/tui-layout.c,
- then gdb/python/py-tui.c.

So when running test-case gdb.python/tui-window-factory.exp, we see the
following order of events:
- the destructor for known_window_types is called, which triggers calling the
  destructor for the only element E of m_window_maker_list.  The destructor
  destroys E, and also removes E from m_window_maker_list, leaving it empty.
- the destructor for m_window_maker_list is called.  It's empty, so it's a nop.

However, when building gdb with -O2 -flto=auto, the static destructor calling
order seems to be reversed.

Instead, we have these events:
- the destructor for m_window_maker_list is called.  This doesn't destroy it's
  only element E, but it does make m_window_maker_list empty.
- the destructor for known_window_types is called, which triggers calling the
  destructor for E.  An attempt is done to remove E from m_window_maker_list,
  but we run into an assertion failure, because the list is empty.

Fix this by checking is_linked () before attempting to remove from
m_window_maker_list, similar to how things were addressed in commit 995a34b1772
("Guard against frame.c destructors running before frame-info.c's").

Tested on x86_64-linux.

PR tui/30646
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30646