Remove user_search_secondary_email feature flag

Catalin Irimierequested to merge
cat-remove-user-secondary-email-ff into master

What does this MR do?

Removes the user_search_secondary_email feature flag and enables searching by secondary email for users by default. It's been enabled on GitLab.com since 2020-11-24.

Related to #282137 (closed).

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