Revert "Reidline mode needs Reline what needs String#grapheme_clusters"

This reverts commit 2b0b19b.

New IRB parser needs new Ripper what has lex_state too. The new Ripper is
adopted by Ruby 2.5 or later.

lib/irb/input-method.rb

@@ -293,5 +293,5 @@ def line(line_no)
     def encoding
       @stdin.external_encoding
     end
-  end if RUBY_VERSION >= "2.5.0" # Reline needs String#grapheme_clusters
+  end
 end