tox-py38 : don't run on Fedora

I'm not sure why I62f6c238de5c55b7673ae8b38a6ededdd77f4d4b added these
with Fedora; maybe it was the only 3.8 environment at the time.  I
don't think we want to run the tests there now.

Change-Id: Iee0e2b60e49e8149b045eaedccff034e5f5a1cfd

.zuul.yaml

@@ -75,15 +75,13 @@
       jobs:
         - zuul-registry-build-image
         - tox-pep8
-        - tox-py38:
+        - tox-py38
-            nodeset: fedora-latest
         - build-python-release
     gate:
       jobs:
         - zuul-registry-upload-image
         - tox-pep8
-        - tox-py38:
+        - tox-py38
-            nodeset: fedora-latest
         - build-python-release
     promote:
       jobs: