stable-only: drop bionic wheel cache job

The ubuntu-bionic nodes are no longer available in the CI, so this job
has been deleted. Clean up the dangling references to avoid zuul config
errors.

Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I2e49dfff6bbc3d7de0681563cd7d93edbc48b167
(cherry picked from commit e7690d7)
(cherry picked from commit e7b93e8)

Author: osfrickler

.zuul.d/project.yaml

@@ -3,9 +3,6 @@
       - openstack-python3-jobs
     check:
       jobs:
-        - build-wheel-cache-ubuntu-bionic:
-            files:
-              - bindep.txt
         - build-wheel-cache-ubuntu-focal:
             files:
               - bindep.txt
@@ -65,9 +62,6 @@
               - bindep.txt
     gate:
       jobs:
-        - build-wheel-cache-ubuntu-bionic:
-            files:
-              - bindep.txt
         - build-wheel-cache-ubuntu-focal:
             files:
               - bindep.txt