metrics: drop release schedule before writing.

After change from dropping entire database to individual measurements this
was overlooked and instead new points are simply written each time which
results in duplicates.

metrics.py

@@ -345,6 +345,7 @@ def ingest_release_schedule(project):
             'time': timestamp(date),
         })
 
+    client.drop_measurement('release_schedule')
     client.write_points(points, 's')
     return len(points)