*scratch1*.el

(defun clear-shell ()
   (interactive)
   (let ((comint-buffer-maximum-size 0))
     (comint-truncate-buffer)))
(define-key shell-mode-map (kbd "C-l") 'clear-shell)

(defun delete-completion-window-buffer (output)
  (interactive)
  (dolist (win (window-list))
    (when (string= (buffer-name (window-buffer win)) "*Completions*")
      (delete-window win)
      (kill-buffer "*Completions*")))
  output)

(add-hook 'comint-preoutput-filter-functions 'delete-completion-window-buffer)