conf.py: remove domainref for workflow examples

Problem: The workflow examples repo has been removed, but it still has
a ref configured in flux-docs conf.py.

Remove it.

Author: grondo

conf.py

@@ -193,10 +193,6 @@
         "https://flux-framework.readthedocs.io/projects/flux-security/en/latest/",
         None,
     ),
-    "workflow-examples": (
-        "https://flux-framework.readthedocs.io/projects/flux-workflow-examples/en/latest/",
-        None,
-    ),
 }
 
 # -- Options for HTML output -------------------------------------------------