Remove invalid annotations from SoftDeletable traits

Class level annotations are being used at the property level, making these traits unusable.


Currently the following error is thrown when validating the schema:
````
[Doctrine\Common\Annotations\AnnotationException]                                                                                                                             
  [Semantical Error] Annotation @Gedmo\SoftDeleteable is not allowed to be declared on property XXXX. You may only use this annotation on these code elements: CLASS.
````

Also the deletedAt field in the Entity trait didn't allow setting deletedAt to null