Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2110345 by mr.york, effulgentsia, Désiré, fgm, fago, pfrenssen, stefan.r, Berdir, Rajendar Reddy: Simplify validation constraint implementations for fields.

Revert "Issue #2002162 by klausi, eugene.ilyin: Convert form validation of users to entity validation."

This reverts commit 5b41b6f8.