Refactor: Remove Mp4Type

[dvdsk]

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.

rodio/src/decoder/mod.rs

Line 314 in 70c236c

pub fn new_mp4(data: R, hint: Mp4Type) -> Result<Decoder<R>, DecoderError> {

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