openshift/os: remove useless build_root image

We don't have a `binary_build_commands` nor use the `pipeline:root`
image so specifying a `build_root` is useless.

Author: jlebon

ci-operator/config/openshift/os/openshift-os-master.yaml

@@ -3,11 +3,6 @@ base_images:
     name: coreos-assembler
     namespace: coreos
     tag: latest
-build_root:
-  image_stream_tag:
-    name: fcos-buildroot
-    namespace: coreos
-    tag: testing-devel
 images:
 - dockerfile_literal: |
     # COSA + source code