RevisionableContentEntityForm creates duplicate log field

When using RevisionableContentEntityForm to help build a revisionable content edit form, the log message field is duplicated if the entity implementation also extends RevisionableContentEntityBase. Specifically, RevisionableContentEntityBase uses RevisionLogEntityTrait, which adds revision_log_message to the form.

Looks to be the same as https://github.com/fago/entity/issues/34 - will submit a PR there.