Stop caching `node_modules` directory

Caching the node_modules directory is inadvisable as the makeup is not always deterministic and incremental runs of yarn install can have unexpected effects (especially when changing versions of yarn).

This will also be problematic as subsequent builds containing different sets of dependencies will each overwrite their changes to the directory within the cache. A far better way to implement caching here is the way it is done in the main gitlab repo:

We cache a directory named .yarn-cache:
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab-ci.yml#L12

.default-cache: &default-cache
  key: "ruby-2.4.4-debian-stretch-with-yarn"
  paths:
    - vendor/ruby
    - .yarn-cache/
    - vendor/gitaly-ruby

And we install our node modules using that directory to seed our yarn cache:
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab-ci.yml#L415

compile-assets:
  <<: *dedicated-runner
  <<: *except-docs
  <<: *use-pg
  stage: prepare
  cache:
    <<: *default-cache
  script:
    - node --version
    - date
    - yarn install --frozen-lockfile --cache-folder .yarn-cache
...