Doctrine\Common\Util\ClassUtils being deprecated

In my application I am using the latest doctrine 2.x. Recently we found it necessary to also use the static function `Doctrine\Common\Util\ClassUtils::getRealClass(string $class)`. But this function is marked as deprecated.

I asked in [issue 826](https://github.com/doctrine/common/issues/826) what would be the alternative function to use. Then I got the response that there is no alternative to that function in doctrine 2.x.

This issue is about removing those depreaction notices, since I see no point in marking something as deprecated when the application programmers have nothing to replace it with.

See also comment https://github.com/doctrine/common/issues/826#issuecomment-472627100