Remove 24h time setting

Description

The user_time_settings feature flag was used to hide a few time-related settings that were under development. The remaining usage of the feature flag is to hide a checkbox to toggle the 24h time format user setting.

Currently, the time_format_in_24h setting has no effect on the display of time/dates, it's not respected anywhere in the frontend or backend code, additionally doesn't take into account the user's browser locale, so as it stands, removing the user setting at the moment would have no impact. Removing the setting allows us to remove the user_time_settings feature flag.

Current status of the user_time_settings feature flag:

It looks like we don't need to update the documentation https://docs.gitlab.com/ee/administration/timezone.html#changing-time-zone-per-user

Related to #29669 (closed).