Bug 1596102 - Disable vismet on browsertime. r=perftest-reviewers,rwood

This bug disables vismet for browsertime until the png/artifact issues are resolved.

Differential Revision: https://phabricator.services.mozilla.com/D52837

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/raptor.yml

@@ -1038,7 +1038,7 @@ browsertime-tp6-1:
     run-visual-metrics:
         by-app:
             chrome: false
-            default: true
+            default: false
     mozharness:
         extra-options:
             - --browsertime