Remove the longer_count_cache_validity_period feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Remove the longer_count_cache_validity_period FF. This has been enabled on .com since March 23rd: #325458 (comment 535977670)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• [-] Documentation (if required)
• Code review guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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