[Feature flag] Clean up projects_tokens_optional_encryption

Summary

This issue is to handle the clean up of projects_tokens_optional_encryption feature flag.

Owners

• Team: grouprunner in GitLab.org / Quality Department / triage-reports
• Most appropriate slack channel to reach out to: #g_runner
• Best individual to reach out to: @erushton
• PM: @DarrenEastman

Plan

Not much else is known about this feature flag at the moment as it is quite old and was introduced in %11.9. First step is to figure out what needs to happen and how we can move toward removing it. See a bit more context at gitlab-org/quality/triage-reports#3320 (comment 600399342)