Remove docker-pull hack

No longer check if image exists by doing a docker-pull -
binderhub will no longer start a build if it knows that
the image already exists

repo2docker/app.py

@@ -251,25 +251,14 @@ class Repo2Docker(Application):
         s.close()
         return port
 
-    def image_exists(self):
-        client = docker.from_env(version='auto')
-        # HACK: Try to just pull this and see if that works.
-        # if it does, then just bail.
-        # WHAT WE REALLY WANT IS TO NOT DO ANY WORK IF THE IMAGE EXISTS
-        try:
-            image = client.images.pull(self.output_image_spec)
-            return True
-        except docker.errors.ImageNotFound:
-            return False
-
     def start(self):
-        if not self.image_exists():
         checkout_path = os.path.join(self.git_workdir, str(uuid.uuid4()))
         self.fetch(
             self.repo,
             self.ref,
             checkout_path
         )
+
         for bp_class in self.buildpacks:
             bp = bp_class(parent=self, log=self.log, capture=self.json_logs)
             if bp.detect(checkout_path):
@@ -288,6 +277,3 @@ class Repo2Docker(Application):
 
         if self.run:
             self.run_image()
-
-
-