Remove deprecated test.related_bug decorator

All tempest plugin now switched to use related_bug
from tempest.lib.decorators
Last patch is proposed in mistral to use the lib interface
which is depends-on of this patch.

We can remove deprecated tempest.test.related_bug.
Depends-On: I20497d48a46195644a114dd9f6b86aca59303097

Change-Id: Ic9ea144750a216d15a372c390d3160934220d2f9
diff --git a/tempest/test.py b/tempest/test.py
index 317c0a7..9f433d5 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -44,11 +44,6 @@
     version='Mitaka', removal_version='?')
 
 
-related_bug = debtcollector.moves.moved_function(
-    decorators.related_bug, 'related_bug', __name__,
-    version='Pike', removal_version='?')
-
-
 attr = debtcollector.moves.moved_function(
     decorators.attr, 'attr', __name__,
     version='Pike', removal_version='?')