Remove feature flag call that guarded nothing

What does this MR do?

It was discovered that this specific FF call guarded nothing (#29669 (comment 620049655)), and so we inadvertently released ability to set user timezone preference.

As development feature flags are supposed to be short-lived, let's keep what's already released, and keep moving forward.

Also, https://about.gitlab.com/handbook/values/#low-level-of-shame

Related issues: #29669 (closed), #15137 (closed)

/cc @ekigbo @peterhegman

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