Various clean up and refactor to make mediawiki-core-npm faster

* Switch all jobs for hasNpm nodes to use git-remoteonly-zuul

  Currently all jobs for slaves in labs (label hasNpm) throw an error
  trying to fetch from /srv/ssd/gerrit.

  Examples:
  - https://integration.wikimedia.org/ci/job/mediawiki-core-npm/5/console

  - https://integration.wikimedia.org/ci/job/mwext-VisualEditor-npm/942/console
    21:44:57 ERROR: Reference path does not exist: /srv/ssd/gerrit/mediawiki/extensions/VisualEditor.git

  - https://integration.wikimedia.org/ci/job/oojs-core-npm/19/consoleFull
    23:35:30 ERROR: Reference path does not exist: /srv/ssd/gerrit/oojs/core.git

  This change creates a separate defaults/git scm that doesn't use local
  ssd/gerrit ("git-remoteonly-zuul"), nor ssd/gerrit with url, but only the url method.

* Use git-clean for all scm's, not just mediawiki-core to speed up
  jobs by not re-cloning repositories.

  We already did this for the git-mwcore scm, but because that one uses /srv/ssd/gerrit
  we can't use that for hasNpm. Re-cloning all of mediawiki-core on hasNpm each
  job takes 3-4 minutes easily. Speed this up by doing a git-clean instead of
  a full workspace wipe.

  Since we've been doing this for mwcore for a while now and haven't seen any
  side effects (git-clean is pretty good at what it does) figured I'd just enable
  this for all jobs instead of trying to hack a way to do it just for mwcore-npm.

Misc clean up:

* There was one extra scm still hardcoded in defaults.yaml, moved that
  one to scm.yaml as well as 'git-localonly-zuul'. Now localonly, remote and
  remoteonly are all together in scm.yaml.

Change-Id: I9aa1aabdd20737881bcf51e78c5d05b6e5270f15