Presently, the strict filename or frontmatter are not being used; so we're validating them unnecessarily. So, for now, remove these checks until we decide to either do something with them, or delete the code entirely.
This reduces duplicate code between `assemble` and `validate`. It also has the benefit of fixing a bug in the current implementation of `validate`, where `isValidChangenoteFile` receives a relative file name where it should receive an absolute one.
