[Feature flag] Remove user setting time preferences

What

Remove the :user_time_settings feature flag:

• time_format_in_24h profile setting
• time_display_relative profile setting Done in !65570 (merged)
• timezone profile setting Done in !65605 (merged)

Next steps

• Remove time_format_in_24h code and user_time_settings feature flag !119025 (merged)
• Review docs https://docs.gitlab.com/ee/administration/timezone.html#changing-time-zone-per-user
• Drop time_format_in_24h database column, docs
  • Ignore the column %16.0 !119025 (merged)
  • Drop the column %16.1 !121148 (merged)
  • Remove ignore rule %16.2
• Feature flag only exists on staging /chatops run feature delete user_time_settings --dev --staging --staging-ref --production

Expectations

What are we expecting to happen?

Users will be able to set their time preferences as per gitlab-ce!25381 and gitlab-ce#37900

What might happen if this goes wrong?

Time data across the application displays incorrectly