Follow-up: Update Api.getRawFile to make ref param optional

Problem to solve

With #323450 (closed) the ref param will become optional and default to the default branch in the backend, this means that the getRawFile method in api.js should no longer default ref to any value to make it an optional parameter.

Proposed solution

Remove { ref: 'master' } from getRawFile and update all of the method's usage where ref isn't defined and the master branch is expected.