Remove overridden `link_to` helper

The purpose for this override is now handled by `ExternalLinkFilter`.

app/helpers/application_helper.rb

@@ -214,39 +214,6 @@ module ApplicationHelper
     Gitlab::MarkupHelper.asciidoc?(filename)
   end
 
-  # Overrides ActionView::Helpers::UrlHelper#link_to to add `rel="nofollow"` to
-  # external links
-  def link_to(name = nil, options = nil, html_options = {})
-    if options.kind_of?(String)
-      if !options.start_with?('#', '/')
-        html_options = add_nofollow(options, html_options)
-      end
-    end
-
-    super
-  end
-
-  # Add `"rel=nofollow"` to external links
-  #
-  # link         - String link to check
-  # html_options - Hash of `html_options` passed to `link_to`
-  #
-  # Returns `html_options`, adding `rel: nofollow` for external links
-  def add_nofollow(link, html_options = {})
-    begin
-      uri = URI(link)
-
-      if uri && uri.absolute? && uri.host != Gitlab.config.gitlab.host
-        rel = html_options.fetch(:rel, '')
-        html_options[:rel] = (rel + ' nofollow').strip
-      end
-    rescue URI::Error
-      # noop
-    end
-
-    html_options
-  end
-
   def promo_host
     'about.gitlab.com'
   end

spec/helpers/application_helper_spec.rb

@@ -203,43 +203,6 @@ describe ApplicationHelper do
     end
   end
 
-  describe 'link_to' do
-    it 'should not include rel=nofollow for internal links' do
-      expect(link_to('Home', root_path)).to eq('<a href="/">Home</a>')
-    end
-
-    it 'should include rel=nofollow for external links' do
-      expect(link_to('Example', 'http://www.example.com')).
-        to eq '<a href="http://www.example.com" rel="nofollow">Example</a>'
-    end
-
-    it 'should include rel=nofollow for external links and honor existing html_options' do
-      expect(link_to('Example', 'http://www.example.com', class: 'toggle', data: {toggle: 'dropdown'}))
-        .to eq '<a class="toggle" data-toggle="dropdown" href="http://www.example.com" rel="nofollow">Example</a>'
-    end
-
-    it 'should include rel=nofollow for external links and preserve other rel values' do
-      expect(link_to('Example', 'http://www.example.com', rel: 'noreferrer'))
-        .to eq '<a href="http://www.example.com" rel="noreferrer nofollow">Example</a>'
-    end
-
-    it 'should not include rel=nofollow for external links on the same host as GitLab' do
-      expect(Gitlab.config.gitlab).to receive(:host).and_return('example.foo')
-      expect(link_to('Example', 'http://example.foo/bar')).
-        to eq '<a href="http://example.foo/bar">Example</a>'
-    end
-
-    it 'should not raise an error when given a bad URI' do
-      expect { link_to('default', 'if real=1 RANDOM; if real>1 IDLHS; if real>500 LHS') }.
-        not_to raise_error
-    end
-
-    it 'should not raise an error when given a bad mailto URL' do
-      expect { link_to('email', 'mailto://foo.bar@example.es?subject=Subject%20Line') }.
-        not_to raise_error
-    end
-  end
-
   describe 'render_markup' do
     let(:content) { 'Noël' }