Remove Third-party.txt file (opensearch-project#3907)

### Description
The Security plugin is one of only two plugins (alongside the SQL
plugin) which has its own THIRD-PARTY.txt file. This file is not
automatically generated and serves no obvious use. This changes follows
the suggestions on the linked issue (below) and removes the file.

I requested a legal review for this change just in case and will update
this PR with a comment once I have confirmation it is fine to merge.

### Issues Resolved
- resolves opensearch-project#1833


### Check List
- [ ] ~New functionality includes testing~
- [ ] ~New functionality has been documented~
- [x] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

Signed-off-by: Stephen Crawford <[email protected]>