Remove nuxeo-tree-snapshot web ui module

This NXP-21356 task has never been achieved and the work done is now outdated and would need to be merged into Web UI repo (NXP-26830) and aligned on Polymer 3 (NXP-26767).

Moreover, the https://github.com/nuxeo/marketplace-tree-snapshot/blob/master/marketplace/src/main/assemble/assembly.xml file has never been adapted to deploy this new nuxeo-tree-snapshot web ui module.

We are gonna remove this unused web ui bundle. A new task will have to be created if we need a tree snapshot addon for Web UI and probably specs will have to be verified...