bug#70784: Abolish string resizing

[Stefan Monnier]

> Also, I'm leaning toward removing `store-substring` from the manual because
> while it probably felt like a good addition at the time, circumstances have
> changed and it has hardly seen any use at all. (We can mark it obsolete or
> keep it indefinitely but it doesn't really deserve top billing these days.)

Hmm... wasn't aware of this one.
AFAIK between Emacs, GNU ELPA, and NonGNU ELPA, I can see it used in only
two files:

    elpa/packages/elisp-benchmarks/benchmarks/dhrystone.el
    elpa/packages/mmm-mode/mmm-cmds.el

Deprecation sounds about right.


        Stefan