mruby-io/test: remove test for internal IO#_read_buf

mrbgems/mruby-io/test/io.rb

@@ -304,29 +304,6 @@ assert('IO.sysopen, IO#syswrite') do
   io.close
 end
 
-assert('IO#_read_buf') do
-  fd = IO.sysopen $mrbtest_io_rfname
-  io = IO.new fd
-  def io._buf
-    @buf
-  end
-  msg_len = $mrbtest_io_msg.size
-  assert_equal '', io._buf
-  assert_equal $mrbtest_io_msg, io._read_buf
-  assert_equal $mrbtest_io_msg, io._buf
-  assert_equal 'mruby', io.read(5)
-  assert_equal 5, io.pos
-  assert_equal msg_len - 5, io._buf.size
-  assert_equal $mrbtest_io_msg[5,100], io.read
-  assert_equal 0, io._buf.size
-  assert_raise EOFError do
-    io._read_buf
-  end
-  assert_equal true, io.eof
-  assert_equal true, io.eof?
-  io.close
-end
-
 assert('IO#ungetc') do
   io = IO.new(IO.sysopen($mrbtest_io_rfname))
   assert_equal 'm', io.getc