Deleting old `:version` from defcustoms (was: master b76cdd0: Delete libraries obsolete since 23.1 and 23.2)

[Stefan Monnier]

>     Delete libraries obsolete since 23.1 and 23.2

Thanks Stefan.

I did a quick `grep` to see the functions/vars that were obsoleted in
the same time frame, and I saw instead a deluge of

    :version "23.1"

If we consider those thingies old enough to remove their obsolete
functions, shouldn't we also remove the corresponding `:version`
thingies from `defcustom`s?

I see:
- 1 defcustom with a :version of 19.29
- >200 defcustoms with a :version of 20.x
- a bit less than 200 defcustoms with a :version of 21.x
- >400 defcustoms with a :version of 22.x
- >200 defcustoms with a :version of 23.x


-- Stefan