Make sure unneeded fields are not present

[greg0ire]

Instead of ensuring every mapping array has a mappedBy and an inversedBy field, let us do the opposite, and remove them when they are null. Likewise if there is a joinColumns field, it is useless if null or empty.

Found while working on #10681
These unneeded fields make the work unnecessarily hard.

[SenseException]

Why removing the array elements in ClassMetadataBuilderTest when they don't have an impact in _validateAndCompleteAssociationMapping anymore?

[greg0ire]

The array elements in that tests are compare used to build DTOs that we compare to other DTO built thanks to this class, which no longer includes them now. Wait no that's not it… I don't remember why I did this 🤔

Maybe that's something I did before extracting this to a separate commit 🤔

Ah yes that's it! When I use git restore --source=origin/3.0.x tests/Doctrine/Tests/ORM/Mapping/ClassMetadataBuilderTest.php on the branch from #10681 , then the tests fail. The new, reworked hierarchy understands this is not normal:

OutOfRangeException: Unknown property mappedBy on class Doctrine\ORM\Mapping\ManyToOneAssociationMapping

That's what forced me to do this change, but this is the correct thing to do IMO, regardless of whether the test suite passes or not.