Remove unused .contributed-projects class

While working on #13401 and trying to add a new tab to the user profile
page, I came across this. I noticed that the `contributed-projects`
class was only being used in order to select the div in the tests. For
consistency with the other tabs, I decided to remove this class and use
the div's id for the selector.

app/views/users/show.html.haml

@@ -98,7 +98,7 @@
       #groups.tab-pane
         - # This tab is always loaded via AJAX
 
-      #contributed.contributed-projects.tab-pane
+      #contributed.tab-pane
         - # This tab is always loaded via AJAX
 
       #projects.tab-pane

features/steps/user.rb

@@ -28,7 +28,7 @@ class Spinach::Features::User < Spinach::FeatureSteps
   end
 
   step 'I should see contributed projects' do
-    page.within '.contributed-projects' do
+    page.within '#contributed' do
       expect(page).to have_content(@contributed_project.name)
     end
   end