[DeadCode] Remove whole assign statement on unused createStub() in tests

[TomasVotruba]

SideEffectNodeDetector already treats createMock() inside a PHPUnit\Framework\TestCase as side-effect free, so RemoveUnusedVariableAssignRector removes the whole statement. createStub() was not covered, so only the variable was dropped and a dangling call was left behind.

Before:

 final class SomeTest extends TestCase
 {
     public function test()
     {
-        $user = $this->createStub(User::class);
+        $this->createStub(User::class);
     }
 }

After:

 final class SomeTest extends TestCase
 {
     public function test()
     {
-        $user = $this->createStub(User::class);
     }
 }