Remove the `keystroke` function after Mousetrap upgrade

The keystroke function in common_utils.js was added in !28914 (merged). We already ship with a library, Mousetrap, that handles keyboard shortcuts, but it is outdated and doesn't support capturing keyboard events over a single element, like a textarea. Working around this limitation would have been messy, so keystroke() was introduced instead.

#63182 (moved) proposes upgrading Mousetrap to the latest version. Then, we should remove keystroke() and make the markdown editor use Mousetrap to capture keyboard shortcuts instead.