Remove `string-slice' -- it's not very well defined

* doc/lispref/strings.texi (Creating Strings): Ditto.

* lisp/emacs-lisp/subr-x.el (string-slice): Remove.

lisp/emacs-lisp/shortdoc.el

@@ -154,9 +154,6 @@ There can be any number of :example/:result elements."
    :eval (split-string "foo bar")
    :eval (split-string "|foo|bar|" "|")
    :eval (split-string "|foo|bar|" "|" t))
-  (string-slice
-   :eval (string-slice "foo-bar" "-")
-   :eval (string-slice "foo-bar--zot-" "-+"))
   (string-lines
    :eval (string-lines "foo\n\nbar")
    :eval (string-lines "foo\n\nbar" t))

lisp/emacs-lisp/subr-x.el

@@ -310,19 +310,6 @@ than this function."
 If OMIT-NULLS, empty lines will be removed from the results."
   (split-string string "\n" omit-nulls))
 
-(defun string-slice (string regexp)
-  "Split STRING at REGEXP boundaries and return a list of slices.
-The boundaries that match REGEXP are included in the result.
-
-Also see `split-string'."
-  (if (zerop (length string))
-      (list "")
-    (let ((i (string-match-p regexp string 1)))
-      (if i
-          (cons (substring string 0 i)
-                (string-slice (substring string i) regexp))
-        (list string)))))
-
 (defun string-pad (string length &optional padding start)
   "Pad STRING to LENGTH using PADDING.
 If PADDING is nil, the space character is used.  If not nil, it