Allow nil as shortcut character in frequently used features

This results in no entry being defined in the corresponding hydra.

site-lisp/db-customize.el

@@ -153,9 +153,11 @@ them.  Can be used in application shortcuts such as
 `db/helm-shortcuts’.  Each entry is a list of three items: a
 short description, a shortcut character, and the function to
 call.  Customizing this variable redefines the global
-`hydra-shortcuts' mapping."
+`hydra-feature-shortcuts'.  Instead of a shortcut character, nil
+can be chosen, in which case no entry in the
+`hydra-feature-shortcuts' will be generated."
   :group 'personal-settings
-  :type  '(repeat (list string character function))
+  :type  '(repeat (list string (choice character (const nil)) function))
   :set #'(lambda (symbol value)
            (set-default symbol value)
            ;; Update hydra when already possible available

site-lisp/db-hydras.el

@@ -60,14 +60,19 @@ _h_   _l_   _o_k        _y_ank
 ;; `db/frequently-used-features'.
 
 (defun db/define-feature-shortcuts-hydra ()
-  "Globally define `hydra-feature-shortcuts' for feature shortcuts."
+  "Globally define `hydra-feature-shortcuts' for feature shortcuts.
+If instead of a shortcut character nil is provided, no entry in
+the hydra will be generated.  See documentation of
+`db/frequently-used-features' for details."
   (eval
    `(defhydra hydra-feature-shortcuts (:color blue)
       ""
       ,@(mapcar (lambda (entry)
                   (pcase-let ((`(,description ,shortcut ,function) entry))
                     (list (string shortcut) function description)))
-                db/frequently-used-features))))
+                (cl-remove-if #'(lambda (entry)
+                                  (null (cl-second entry)))
+                              db/frequently-used-features)))))
 
 (db/define-feature-shortcuts-hydra)