bug#53910: [External] : bug#53910: 29.0.50; context-menu-mode breaks help in read-only buffers

[Juri Linkov]

>> If `context-menu-mode` is intended to work only on displayed buffers,
>> is it really important to ensure that it also doesn't fail on
>> non-window buffers?
>
> Yes, a :filter function in a keymap should not make such assumptions
> (nor have side-effects, as a general rule).

I can't find a way to prevent :filter from executing on context menus in help
buffers, so I just removed `help-buffer-under-preparation` from :filter.