fix: use USNs directly from the CVE response

We need to fetch the USNs related to a CVE to extract the ubuntu
version of the packages affected by the CVE. In the past, we
where running another request to fetch all USNs related to the CVEs.
To make this process less demanding on the API side, we are now
relaying on the related USNs already delivered by the CVE endpoint
instead

uaclient/api/u/pro/security/fix/_common/plan/v1.py

@@ -742,7 +742,7 @@ def _get_cve_data(
 ) -> Tuple[CVE, List[USN]]:
     try:
         cve = client.get_cve(cve_id=issue_id)
-        usns = client.get_notices(cves=issue_id)
+        usns = cve.notices
     except exceptions.SecurityAPIError as e:
         if e.code == 404:
             raise exceptions.SecurityIssueNotFound(issue_id=issue_id)