Mark classes as final

[malarzm]

Several classes are now marked as final. Some things I was considering:

• is class marked as internal
• does it have an interface
• is there any way one could extend the class and use somehow inject it into ODM again

By the way I've removed two unused mock classes and a test that was testing nothing (interesting) and allowed me to make ClassMetadataFactory final

[alcaeus]

With 1.3.x now being merged down into master, we no longer prepare separate pull requests to prepare deprecations. The course of action in this case would be to create a pull request to 1.3.x that marks the classes in question as @final and throws a deprecation when a class is extended. To do so, you can compare self:class against static::class: https://3v4l.org/jE9Ef.

Once the PR to 1.3.x has been merged and the branch merged into master, this PR can be rebased to drop the @final annotations and make the classes final for good.

[alcaeus]

#2008 was merged into 1.3.x and further on to master, so this can be rebased. @malarzm you can remove changes to phpstan.neon.dist - I had to silence errors related to @final to get it stable: ba0798e#diff-0847156bd64b0de0f4d91da2394ec0e8R11.