bug#20732: in-string-p fails

[Drew Adams]

> > I.e. remove all uses and declare it obsolete.  Sounds perfect.
> 
> Done. Guess I'll keep its definition as-is; maybe some people had a
> reason to use it.

I use it in thingatpt+.el:

(let (strg-end)
  (while (setq strg-end  (in-string-p))
    ;; Skip past string element of list.
    (skip-syntax-forward "^\"")
    ;; Skip past new string opening, `"', into next string.
    (skip-syntax-forward "\"")))

Please be sure to specify how `in-string-p' should be replaced.