Revert "fix ConcurrentModificationException"

This reverts commit 27705e31f429b68458586526bb100a4cdfc53568.
Revert "Spoofax/851"

This reverts commit 57cc402632e50b8749b84c1b6eba3487a9542d70.
fix ConcurrentModificationException once and for all https://community.oracle.com/message/9027285