Refactor: Remove Mp4Type

[yara-blue]

Mp4 extensions do not have any real value: https://en.wikipedia.org/wiki/MP4_file_format#Filename_extensions

Therefore the mp4 constructor can just pass mp4. https://github.com/RustAudio/rodio/blob/70c236c7ac495b9ccb82716ffba06cb3fe5fa44c/src/decoder/mod.rs#L314

This makes the function more in line with the rest (no need for a hint) and removes confusion from users.