(https://github.com/ruby/irb/pull/574)

`MagicFile` was introduced around v0.9.6, which was like 14~15 years ago.
It was needed because back then we needed to read a file's magic comment
to determine the encoding of it, and read it with that encoding.

Commit: 3ee79e89

But now we expect files to be encoded in UTF-8 and don't specify encoding
through magic comments anymore, `MagicFile` can be retired.