delete deprecated IO-like methods

This commit deletes
{IO,ARGF,StringIO,Zib::GZipReader}#{bytes,chars,lines,codepoints}, which
have been deprecated since c47c095b97.

Note that String also has those methods.  They are neither depreacted
nor deleted because they are not aliases of counterpart each_something.

spec/ruby/core/io/codepoints_spec.rb

@@ -2,24 +2,27 @@ require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/codepoints'
 
-# See redmine #1667
-describe "IO#codepoints" do
-  it_behaves_like :io_codepoints, :codepoints
-end
+ruby_version_is ''...'2.8' do
 
-describe "IO#codepoints" do
-  before :each do
-    @io = IOSpecs.io_fixture "lines.txt"
+  # See redmine #1667
+  describe "IO#codepoints" do
+    it_behaves_like :io_codepoints, :codepoints
   end
 
-  after :each do
-    @io.close unless @io.closed?
-  end
+  describe "IO#codepoints" do
+    before :each do
+      @io = IOSpecs.io_fixture "lines.txt"
+    end
 
-  it "calls the given block" do
-    r = []
-    @io.codepoints { |c| r << c }
-    r[24].should == 232
-    r.last.should == 10
+    after :each do
+      @io.close unless @io.closed?
+    end
+
+    it "calls the given block" do
+      r = []
+      @io.codepoints { |c| r << c }
+      r[24].should == 232
+      r.last.should == 10
+    end
   end
 end