bug#70139: 29.3.50; key-translate does not support all keyboard-translate usages

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Tue, 02 Apr 2024 11:50:40 +0200
> 
> 
> When `key-translate' was added, it didnʼt cover all the cases that
> `keyboard-translate' does.
> 
> Add a translation:
> 
>     (keyboard-translate ?\C-a ?\C-z)
> 
> Two ways to remove, of which I submit the first is 'obvious':
> 
>     (keyboard-translate ?\C-a nil)
>     (keyboard-translate ?\C-a ?\C-a)
> 
> Add:
> 
>     (key-translate "C-a" "C-z")
> 
> This works for removing a translation but is non-obvious:
> 
>     (key-translate "C-a" "C-a")
> 
> But this doesnʼt:
> 
>     (key-translate "C-a" nil)
> 
> =>
> Debugger entered--Lisp error: (error "nil is not a valid key definition; see ‘key-valid-...")
>   signal(error ("nil is not a valid key definition; see ‘key-valid-..."))
>   error("%S is not a valid key definition; see `key-valid-p..." nil)
> 
> Iʼm not sure this is worth fixing, but perhaps documenting that
> re-adding the same translation is (almost) the same as removing it?

Stefan, any comments or suggestions?