emacs: fix windmove in shell modes

comint-mode-map defines C-up and C-down as comint-previous-input and
comint-next-input. This overrides windmove, since I've told windmove to
use ctrl as its modifier. The functions are also bound to M-p and M-n in
comint-mode, so unbind C-up and C-down from comint-mode-map.