remove source from the FieldPresenceEnhancer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I am not sure when its really useful to also check if a ``source`` field is also present. To me this enhancer is about setting a default for a given field. Why would I only want to set a default for ``_controller`` if ``_template`` is set and ``_controller`` is not set?

/cc @dbu