Remove superfluous inclusion

The EDN::Type module does not need this.

Also, EDN::Type::List already includes EDN::CoreExt::AllowsMetadata
through its parent class, Array.
Merge branch 'remove-superfluous-inclusion' of https://github.com/jgdavey/edn-ruby into jgdavey-remove-superfluous-inclusion
Merge branch 'jgdavey-remove-superfluous-inclusion'