PIM-1771 product repository

- [x] FlexibleEntityRepositoryInterface cleanup, remove useless methods
- [x] FlexibleEntityRepositoryInterface should not expose QueryBuilder (to prepare Mongo impl)

Notice : after/during grid migration (cf https://github.com/akeneo/pim-community-dev/tree/oro/grid), we would remove/change applyFilterByAttribute and applySorterByAttribute methods

Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]
ChangeLog updated: [no]
Fixes the following jira: PIM-1771