This reverts commit 788d2731.

Most usages of AR.js uses artoolkit as backend. It seems
better-supported overall as well.
Dropping the scripts necessary to support aruco as backend would reduce
build size by 72kb (34kb minified).
There are some examples that utilizes aruco still (multi-tracker), so we
don't want to completely cleanse the project of aruco. Therefore we
offer ar.lean.js as additional build choice.