minor #13470 [2.7] Removed dead code and various cleaning (saro0h)

This PR was merged into the 2.7 branch.

Discussion
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[2.7] Removed dead code and various cleaning

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

In complement of these: symfony/symfony#13488 & symfony/symfony#13481

Commits
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9272f86 Removed dead code and various cleaning

Author: fabpot

Constraints/CallbackValidator.php

@@ -11,7 +11,6 @@
 
 namespace Symfony\Component\Validator\Constraints;
 
-use Symfony\Component\Validator\Context\ExecutionContextInterface;
 use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\ConstraintValidator;
 use Symfony\Component\Validator\Exception\ConstraintDefinitionException;

Constraints/NotNullValidator.php

@@ -11,7 +11,6 @@
 
 namespace Symfony\Component\Validator\Constraints;
 
-use Symfony\Component\Validator\Context\ExecutionContextInterface;
 use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\ConstraintValidator;
 use Symfony\Component\Validator\Exception\UnexpectedTypeException;