Remove old way of defining artifacts from browser performance testing templates

First Job/Browser-Performance-Testing.gitlab-ci.yml

Old:

  artifacts:
    paths:
      - performance.json
      - sitespeed-results/

New:

  artifacts:
    reports:
      performance: performance.json

and then on our new vendored template just remove:

    paths:
      - performance.json
      - sitespeed-results/