staging: Deactivate json logging on bzr loader

It cannot use it for now as the image build associated enhanced with the proper deps is
stuck.

Refs. swh/infra/sysadm-environment#4524
Refs. swh/infra/sysadm-environment#4862

swh/values/staging.yaml

@@ -35,7 +35,6 @@ loaders:
         - swh.loader.bzr.tasks.LoadBazaar
       autoScaling:
         maxReplicaCount: 1
-      useJsonLogger: true
     cran:
       requestedMemory: 256Mi
       requestedCpu: 200m