Remove unnecessary :group attributes

Remove unnecessary :group attributes

* which-key.el:  Remove :group 'which-key in defcustoms.
(which-key-idle-delay):
(which-key-idle-secondary-delay):
(which-key-echo-keystrokes):
(which-key-max-description-length):
(which-key-min-column-description-width):
(which-key-add-column-padding):
(which-key-unicode-correction):
(which-key-dont-use-unicode):
(which-key-separator):
(which-key-ellipsis):
(which-key-prefix-prefix):
(which-key-compute-remaps):
(which-key-replacement-alist):
(which-key-allow-multiple-replacements):
(which-key-show-docstrings):
(which-key-highlighted-command-list):
(which-key-special-keys):
(which-key-buffer-name):
(which-key-show-prefix):
(which-key-popup-type):
(which-key-min-display-lines):
(which-key-max-display-columns):
(which-key-side-window-location):
(which-key-side-window-slot):
(which-key-side-window-max-width):
(which-key-side-window-max-height):
(which-key-frame-max-width):
(which-key-frame-max-height):
(which-key-allow-imprecise-window-fit):
(which-key-show-remaining-keys):
(which-key-sort-order):
(which-key-sort-uppercase-first):
(which-key-paging-prefixes):
(which-key-paging-key):
(which-key-use-C-h-commands):
(which-key-show-early-on-C-h):
(which-key-is-verbose):
(which-key-preserve-window-configuration):
(which-key-hide-alt-key-translations):
(which-key-delay-functions):
(which-key-allow-regexps):
(which-key-inhibit-regexps):
(which-key-show-transient-maps):
(which-key-init-buffer-hook):