fix: stop writing decision task route on-push

Due to how `mozilla-extensions` repos are configured in fxci-config,
they no longer have access to write to this path for security reasons.

Solving this properly means re-thinking the whole `mozilla-extensions`
Taskcluster architecture (which I think is long overdue anyway).

But in the short term, removing this route means we should get
unblocked.

Author: ahal

.taskcluster.yml

@@ -171,12 +171,7 @@ tasks:
                                   then:
                                       kind: cron-task
                       routes:
-                          $flatten:
-                              - checks
-                              - $if: 'tasks_for == "github-push"'
-                                then:
-                                    - "index.${trustDomain}.v2.${project}.revision.${head_sha}.taskgraph.decision"
-                                else: []
+                          - checks
                       scopes:
                           $if: 'tasks_for == "github-push"'
                           then: