remove source from the FieldPresenceEnhancer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes, well sort of
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

requires https://github.com/symfony-cmf/Routing/pull/85