Revert "Create dependabot.yml"

This reverts commit 462e8b7.

While keeping everything up-to-date may be desirable, the npm ecosystem
moves too fast for us to keep up with automatic updates without an
automated testing system. Revert this in favor of only receiving alerts
and PRs for security updates using the settings in the GitHub web
interface.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>