Move has_trace? to CommitStatus than GenericCommitStatus

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7617#note_18762721

app/models/commit_status.rb

@@ -135,15 +135,19 @@ class CommitStatus < ActiveRecord::Base
     allow_failure? && (failed? || canceled?)
   end
+  def duration
+    calculate_duration
+  end
+
   def playable?
     false
   end
-  def duration
-    calculate_duration
+  def stuck?
+    false
   end
-  def stuck?
+  def has_trace?
     false
   end
 end

app/models/generic_commit_status.rb

@@ -12,8 +12,4 @@ class GenericCommitStatus < CommitStatus
   def tags
     [:external]
   end
-
-  def has_trace?
-    false
-  end
 end