kestrel_datasource_stixshifter doesn't delete searches

[pcoccoli]

Describe the bug
stix-shifter has a "transmit delete" operation (stix_transmission.DELETE) but the interface doesn't use it. On some data sources (e.g. qradar, gcp_chronicle, bigquery) this should result in old searches/search results being orphaned on the data source.

After fetching all results, we should call the delete operation. Some stix-shifter modules may not implement it, so we need to catch (and ignore or just log) NotImplementedError.

[subbyte]

#460