Remove last reference to six

Change I19d298cf0f402d65f0b142dea0bf35cf992332a9 removed most of these
but missed one.

Change-Id: If7db47b5d93b94337bf8ff141a0e5aa5b88c448a
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tools/speed_test.py

@@ -111,7 +111,7 @@ def main():
     dummy_am = max(0, args.dummies)
     with ctx_manager("Building linear flow with %s tasks" % dummy_am, args):
         f = lf.Flow("root")
-        for i in compat_range(0, dummy_am):
+        for i in range(0, dummy_am):
             f.add(DummyTask(name="dummy_%s" % i))
     with ctx_manager("Loading", args):
         e = engines.load(f)