Retire magic-file.rb (#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: 3ee79e8

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

Author: st0012

lib/irb/help.rb

@@ -4,8 +4,6 @@
 #   	by Keiju ISHITSUKA(keiju@ishitsuka.com)
 #
 
-require_relative 'magic-file'
-
 module IRB
   # Outputs the irb help message, see IRB@Command+line+options.
   def IRB.print_usage

lib/irb/input-method.rb

@@ -5,7 +5,6 @@
 #
 
 require_relative 'src_encoding'
-require_relative 'magic-file'
 require_relative 'completion'
 require 'io/console'
 require 'reline'
@@ -132,7 +131,7 @@ def open(file, &block)
     # Creates a new input method object
     def initialize(file)
       super
-      @io = file.is_a?(IO) ? file : IRB::MagicFile.open(file)
+      @io = file.is_a?(IO) ? file : File.open(file)
       @external_encoding = @io.external_encoding
     end
     # The file name of this input method, usually given during initialization.