Stubs should not be loaded when using versions of Symfony with upstream generic types

[stof]

Once generic types are added upstream in Symfony itself, the stubs should not be loaded again to override them, so that the official type definitions are used.

phpstan-doctrine already has a similar logic to prefer using the official generic types instead of stubbing them.

[stof]

@ondrejmirtes should I submit this to the 1.4.x branch or the 2.0.x branch ?

[ondrejmirtes]

@stof I'd accept either, given the merge would be easy and conflict-free (very likely).