fix: delete duplicate discovery artifacts (#1391)

This PR removes duplicated discovery artifacts which are no longer being updated.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 🦕

googleapiclient/discovery_cache/documents/admin.datatransferv1.json

@@ -0,0 +1 @@
+admin.datatransfer_v1.json

googleapiclient/discovery_cache/documents/admin.directoryv1.json

@@ -0,0 +1 @@
+admin.directory_v1.json

googleapiclient/discovery_cache/documents/admin.reportsv1.json

@@ -0,0 +1 @@
+admin.reports_v1.json