Merge branch 'issues_url' into 'master'

Remove placeholder methods to prevent calling methods rather than attributes.

Since we have https://dev.gitlab.org/gitlab/gitlabhq/blob/7-8-stable/app/models/service.rb#L79 makes no sense to have methods as a placeholder.

See merge request !1676

app/models/project_services/issue_tracker_service.rb

@@ -25,18 +25,6 @@ class IssueTrackerService < Service
     false
   end
-  def project_url
-    # implement inside child
-  end
-
-  def issues_url
-    # implement inside child
-  end
-
-  def new_issue_url
-    # implement inside child
-  end
-
   def issue_url(iid)
     self.issues_url.gsub(':id', iid.to_s)
   end