(minibuffer-completion-help): Do not check completions-group centrally

The guard variable `completions-group` should be checked in each
completion table individually.  The guard variable
`completions-detailed` variable is used in the same way.

* minibuffer.el (minibuffer-completion-help): Remove check of
  `completions-group`.

lisp/minibuffer.el

@@ -2182,8 +2182,7 @@ variables.")
                           (plist-get completion-extra-properties
                                      :affixation-function)))
              (sort-fun (completion-metadata-get all-md 'display-sort-function))
-             (group-fun (and completions-group
-                             (completion-metadata-get all-md 'group-function)))
+             (group-fun (completion-metadata-get all-md 'group-function))
              (mainbuf (current-buffer))
              ;; If the *Completions* buffer is shown in a new
              ;; window, mark it as softly-dedicated, so bury-buffer in