Don't manage platform:f28 in stage MBS

roles/mbs/common/files/default-modules.staging/platform-f28.yaml

@@ -1,27 +0,0 @@
-data:
-  description: Fedora 28 traditional base
-  license:
-    module: [MIT]
-  name: platform
-  profiles:
-    buildroot:
-      rpms: [bash, bzip2, coreutils, cpio, diffutils, fedora-release, findutils, gawk,
-        gcc, gcc-c++, grep, gzip, info, make, patch, redhat-rpm-config, rpm-build,
-        sed, shadow-utils, tar, unzip, util-linux, which, xz]
-    srpm-buildroot:
-      rpms: [bash, fedora-release, fedpkg-minimal, gnupg2, redhat-rpm-config, rpm-build,
-        shadow-utils]
-  stream: f28
-  summary: Fedora 28 traditional base
-  version: 4
-  context: 00000000
-  xmd:
-    mbs:
-      buildrequires: {}
-      commit: virtual
-      requires: {}
-      koji_tag: module-f28-build
-      mse: TRUE
-document: modulemd
-version: 1
-