drop substr in the tree builder, its the frontends job

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | #223
| License       | MIT
| Doc PR        | -

alternative to #223 by @ivan1986 . this depends on https://github.com/symfony-cmf/TreeBrowserBundle/pull/55