[Validator] Remove property and method targets from the optional and required constraints

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |  #11013
| License       | MIT
| Doc PR        | 

At the moment both constraints can only be defined on other annotations. Both constraints are [only mentioned in the docs in the context of the Collection](http://symfony.com/doc/current/reference/constraints/Collection.html#required-and-optional-field-constraints).

Defining the required or optional annotation directly on a field or method 
throws a ClassNotFoundException, since the constraint validator factory tries to load the validator (which does not exist):

```
ClassNotFoundException: Attempted to load class "OptionalValidator"
from namespace "Symfony\Component\Validator\Constraints" 
in /var/www/server/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Validator/ConstraintValidatorFactory.php line 71. 
Do you need to "use" it from another namespace?
```

By applying this patch the end user will get a more helpful error message:

```
[Semantical Error] Annotation @Assert\Optional is not allowed to be declared on property Acme\DemoBundle\Entity\Contact::$message. 
You may only use this annotation on these code elements: ANNOTATION.
```