Remove locking code from workflow manager

The workflow manager got some complicated locking code in REPO-224, that supposedly should have fixed concurrent workflow operations on the same document.
The document model relies on the documents having the same name as the handle. Combining renaming with e.g. editing can then lead to problems in this document model.

However, the implemented solution was to add the 'mix:lockable' mixin to the hippo:hardhandle and always do locking when doing workflow. This last change has been reverted, so no actual locking is being done. The code to do this is all still there. So it is quite dead and upon a closer look contains bugs (instead of unlocking, it locks again in the finally block...).

My proposal is to remove this locking code.