Deneb review .github (CI cleanup) (#4696)

## Issue Addressed

Related to https://github.com/sigp/lighthouse/issues/4676.

Deneb-specifc CI code to be removed before merging to `unstable`. Dot not merge until we're ready to merge into `unstable`, as we may need to release deneb docker images before merging.

Keep in mind that most of the changes in the below PR (to `unstable`) have already 
been merged to `deneb-free-blobs`, so merging `deneb-free-blobs` into `unstable` would include those changes - it would be ok if the release runners are ready, otherwise we may want to exclude them before merging.
- https://github.com/sigp/lighthouse/pull/4592

.github/workflows/docker.yml

@@ -5,7 +5,6 @@ on:
         branches:
             - unstable
             - stable
-            - deneb-free-blobs
         tags:
             - v*
 
@@ -41,11 +40,6 @@ jobs:
               run: |
                     echo "VERSION=latest" >> $GITHUB_ENV
                     echo "VERSION_SUFFIX=-unstable" >> $GITHUB_ENV
-            - name: Extract version (if deneb)
-              if: github.event.ref == 'refs/heads/deneb-free-blobs'
-              run: |
-                    echo "VERSION=deneb" >> $GITHUB_ENV
-                    echo "VERSION_SUFFIX=" >> $GITHUB_ENV
             - name: Extract version (if tagged release)
               if: startsWith(github.event.ref, 'refs/tags')
               run: |