[ticket/12903] Remove dead phpbb\extension\metadata_manager::_validate_version

PHPBB3-12903
Merge pull request #2821 from bantu/ticket/12903

[ticket/12903] Remove dead phpbb\extension\metadata_manager::_validate_version
Merge branch 'develop-ascraeus' into develop

# By jeroendedauw
# Via Andreas Fischer (1) and Dhruv Goel (1)
* develop-ascraeus:
  [ticket/12903] Remove dead phpbb\extension\metadata_manager::_validate_version