Clear unknown checks when recalculating checks in any case

Move delete_unknown_checks to QualityCheck class method
Add check_names param to Unit.update_qualitychecks

- Remove duplicated update_qualitychecks method in refresh_stats.py,
  use Unit method
- Add QUALITY_CHECKER settings param to set default checker for all
  translation projects

according comment in #116
Delete inactive false-positive checks

After codebase changing some false-positive checks can become inactive,
they shoud be deleted