fix: remove duplicate variable declaration (#503)

Co-authored-by: minherz <minherz@users.noreply.github.com>

Author: iennae

samples/snippets/v3/cloud-client/snippets.py

@@ -89,8 +89,6 @@ def list_time_series(project_id):
 
     client = monitoring_v3.MetricServiceClient()
     project_name = f"projects/{project_id}"
-    interval = monitoring_v3.TimeInterval()
-
     now = time.time()
     seconds = int(now)
     nanos = int((now - seconds) * 10 ** 9)