build: drop reference to workflow-examples

Problem: the sphinx build emits warnings about missing
workflow-examples.

loading intersphinx inventory from https://flux-framework.readthedocs.io/projects/flux-workflow-examples/en/latest/objects.inv...
WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://flux-framework.readthedocs.io/projects/flux-workflow-examples/en/latest/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 404 Client Error: Not Found for url: https://flux-framework.readthedocs.io/projects/flux-workflow-examples/en/latest/objects.inv

But that readthedocs project was deleted.

Drop the reference from conf.py.

doc/conf.py

@@ -154,8 +154,4 @@ def setup(app):
         "https://flux-framework.readthedocs.io/projects/flux-rfc/en/latest/",
         None,
     ),
-    "workflow-examples": (
-        "https://flux-framework.readthedocs.io/projects/flux-workflow-examples/en/latest/",
-        None,
-    ),
 }