Remove custom Lexer. #3945 [ci skip]

Inline diff is going to be generated client side now. #3945

lib/gitlab/diff/highlight.rb

@@ -95,8 +95,7 @@ module Gitlab
       end
 
       def lexer
-        parent = Rouge::Lexer.guess(filename: @file_name, source: @code).new rescue Rouge::Lexers::PlainText.new
-        Rouge::Lexers::GitlabDiff.new(parent_lexer: parent)
+        Rouge::Lexer.guess(filename: @file_name, source: @code).new rescue Rouge::Lexers::PlainText.new
       end
 
       def unescape_html(content)

lib/rouge/lexers/gitlab_diff.rb

@@ -1,26 +0,0 @@
-Rouge::Token::Tokens.token(:InlineDiff, 'idiff')
-
-module Rouge
-  module Lexers
-    # This new Lexer is required in order to avoid the inline diff markup
-    # to be tokenized, it will be rendered as raw HTML code if that happens.
-    class GitlabDiff < RegexLexer
-      title "GitLab Diff"
-      tag 'gitlab_diff'
-
-      state :root do
-        rule %r{<span class='idiff'>(.*?)</span>} do |match|
-          token InlineDiff, match[1]
-        end
-
-        rule /(?:(?!<span class='idiff').)*/m do
-          delegate option(:parent_lexer)
-        end
-      end
-
-      start do
-        option(:parent_lexer).reset!
-      end
-    end
-  end
-end