[ci_yaml] Remove back_fill=false on roller (#137076)

Follow up from https://flutter-review.googlesource.com/c/50200 which removed the issue with inconsistencies

flutter/flutter#112127

Author: Casey Hillers

.ci.yaml

@@ -348,7 +348,6 @@ targets:
     properties:
       tags: >
         ["framework", "hostonly", "shard", "linux"]
-      backfill: "false"
     runIf:
       - .ci.yaml