Remove item.layout

We're using item.current_build_set.layout everywhere; it looks like
this isn't actually needed.

Change-Id: I5b16f63a8e8092c25a80d3eac9868be3893e89f6
diff --git a/zuul/manager/__init__.py b/zuul/manager/__init__.py
index e59ad84..5a3f5c4 100644
--- a/zuul/manager/__init__.py
+++ b/zuul/manager/__init__.py
@@ -738,8 +738,7 @@
         # pipeline, use the dynamic layout if available, otherwise,
         # fall back to the current static layout as a best
         # approximation.
-        layout = (item.layout or
-                  item.current_build_set.layout or
+        layout = (item.current_build_set.layout or
                   self.pipeline.layout)
 
         if not layout.hasProject(item.change.project):
diff --git a/zuul/model.py b/zuul/model.py
index 5eedc75..8d2a8d1 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -1339,7 +1339,6 @@
         self.quiet = False
         self.active = False  # Whether an item is within an active window
         self.live = True  # Whether an item is intended to be processed at all
-        self.layout = None  # This item's shadow layout
         self.job_graph = None
 
     def __repr__(self):