[ruby/irb] Drop src_encoding.rb

(https://github.com/ruby/irb/pull/634)

Its method `IRB.default_src_encoding` was only used in `magic-file.rb`,
which has been removed.

https://github.com/ruby/irb/commit/ed98983b1f

lib/irb/input-method.rb

@@ -4,7 +4,6 @@
 #   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
 #
 
-require_relative 'src_encoding'
 require_relative 'completion'
 require 'io/console'
 require 'reline'

lib/irb/src_encoding.rb

@@ -1,7 +0,0 @@
-# frozen_string_literal: false
-# DO NOT WRITE ANY MAGIC COMMENT HERE.
-module IRB
-  def self.default_src_encoding
-    return __ENCODING__
-  end
-end