Cleanup unused code

* jupyter-monads: Remove the `thunk` require, it's no longer used.
Remove the `jupyter-monads` customization group as there are no
customizable variables.

jupyter-monads.el

@@ -40,17 +40,12 @@
 ;;; Code:
 
 (require 'jupyter-base)
-(require 'thunk)
 
 (declare-function jupyter-handle-message "jupyter-client")
 (declare-function jupyter-kernel-io "jupyter-client")
 (declare-function jupyter-generate-request "jupyter-client")
 (declare-function jupyter-wait-until-idle "jupyter-client" (req &optional timeout progress-msg))
 
-(defgroup jupyter-monads nil
-  "Monadic Jupyter"
-  :group 'jupyter)
-
 (defconst jupyter--return-nil (lambda (state) (cons nil state)))
 
 (defun jupyter-return (value)