Don't remove builds that don't exist

We saw test_reconfigure_window_fixed test fail in the change to
switch to Ansible 2.4. This may have been a timing bug where we
try to remove a build that we've already removed.

Change-Id: Ic55eeb35589b6469b0dcce8267f49eda852a49f7

zuul/model.py

@@ -1386,6 +1386,8 @@ class BuildSet(object):
         build.build_set = self
 
     def removeBuild(self, build):
+        if build.job.name not in self.builds:
+            return
         self.tries[build.job.name] += 1
         del self.builds[build.job.name]