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

At the moment both constraints can only be defined on other annotations (specifically, the Collection annotation). 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).
bug #11014 [Validator] Remove property and method targets from the optional and required constraints (jakzal)

This PR was merged into the 2.3 branch.

Discussion
----------

[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.
```

Commits
-------

9c2616e [Validator] Remove property and method targets from the optional and required constraints.
updated CHANGELOG for 2.3.16
update CONTRIBUTORS for 2.3.16
updated VERSION for 2.3.16