Remove PHPUnit_Framework_MockObject_MockObject

[sebastianbergmann]

In time for the release of PHPUnit 8:

• Rename PHPUnit_Framework_MockObject_MockObject to PHPUnit\Framework\MockObject\MockObject
• Remove src/ForwardCompatibility/MockObject.php

This is a BC break (that should have happened together with PHPUnit 7) because many developers use annotations such as

/** @var Foo|PHPUnit_Framework_MockObject_MockObject $foo*/
$foo = $this->createMock(Foo::class);

in their code.