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

[Robert Pluim]

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?

Thanks

Robert

In GNU Emacs 29.3.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.16.0) of 2024-04-02 built on rltb
Repository revision: 6b8b0a12333afeadb32744ba481679b05b758ed2
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Debian GNU/Linux 12 (bookworm)

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