[Security Solution] remove old and skipped Threat Intelligence Cypress tests

[PhilippeOberti]

Summary

This PR is a follow up of this one that migrated all the Threat Intelligence Cypress tests to the security_solution_cypress folder.
It focuses on:

• removing the old tests, the entire cypress folder under the x-pack/threat_intelligence location
• updating the CONTRIBUTING and README files

[elasticmachine]

Pinging @elastic/security-threat-hunting-investigations (Team:Threat Hunting:Investigations)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 1805fc7

Failed CI Steps

• Jest Integration Tests #5

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
@kbn/test-suites-xpack	721	720	-1
threatIntelligence	7	5	-2
total			-3

Total ESLint disabled count

id	before	after	diff
@kbn/test-suites-xpack	745	744	-1
threatIntelligence	7	5	-2
total			-3

History

• 💔 Build #233456 failed dd15928
• 💔 Build #233306 failed 7c8a5c4
• 💔 Build #233302 failed 858cac1

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream