doctrine: Deleting objects with mutual references

[cruzccl]

Remove the temporary hack introduced in dd0559caaa8baefe79e4591b7ba813e9e93c827c

This hack was introduced in 2.0 to solve the problem of Doctrine when deleting an object and this object has a reference to another object that also has a reference to the first object. Before this hack when an user was deleted and it's extension was in memory, doctrine was identifying extension#user as a new entity that was required to be persisted.

This issue aims to solve this problem rewriting some logic of this process.