This PR removes duplicated discovery artifacts which are no longer being updated. I also created symbolic links for the following documents:

admin.datatransferv1.json -> admin.datatransfer_v1.json
admin.directoryv1.json -> admin.directory_v1.json
admin.reportsv1.json -> admin.reports_v1.json

I used the following code to test that the link is working correctly, for admin.datatransferv1.json as an example.

from googleapiclient.discovery import build
ad = build('admin', 'datatransferv1')
print('Built {}.{}'.format(ad._resourceDesc['name'], ad._resourceDesc['version']))

Fixes #1330 🦕