[ruby/irb] Retire magic-file.rb

(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.

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 @@ module IRB
     # 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.

lib/irb/magic-file.rb

@@ -1,38 +0,0 @@
-# frozen_string_literal: false
-module IRB
-  class << (MagicFile = Object.new)
-    # see parser_magic_comment in parse.y
-    ENCODING_SPEC_RE = %r"coding\s*[=:]\s*([[:alnum:]\-_]+)"
-
-    def open(path)
-      io = File.open(path, 'rb')
-      line = io.gets
-      line = io.gets if line[0,2] == "#!"
-      encoding = detect_encoding(line)
-      internal_encoding = encoding
-      encoding ||= IRB.default_src_encoding
-      io.rewind
-      io.set_encoding(encoding, internal_encoding)
-
-      if block_given?
-        begin
-          return (yield io)
-        ensure
-          io.close
-        end
-      else
-        return io
-      end
-    end
-
-    private
-    def detect_encoding(line)
-      return unless line[0] == ?#
-      line = line[1..-1]
-      line = $1 if line[/-\*-\s*(.*?)\s*-*-$/]
-      return nil unless ENCODING_SPEC_RE =~ line
-      encoding = $1
-      return encoding.sub(/-(?:mac|dos|unix)/i, '')
-    end
-  end
-end