Remove Elasticsearch 6 Nightly job

What does this MR do?

We're removing Elasticsearch 6 compatibility testing for the following reasons:

• we are no longer going to support version 6 in 15.0
• we plan to merge this MR gitlab-org/gitlab!79949 (merged) next week. The MR will update the Elasticsearch client and the new client is not backwards compatible with version 6.
• Version 6 tests are currently failing and the Search Team doesn't plan to allocate time and resources to troubleshoot the failures on a soon to obsolete version.
• We no longer support version 6 as of 14.8 https://docs.gitlab.com/ee/integration/elasticsearch.html#version-requirements

Check-list

• Verify the change in all affected pipelines.