settings: Migrate twenty_four_hour_time setting to RealmUserDefault.

This commit removes the existing default_twenty_four_hour_time field in
Realm table which was used to set the twenty_four_hour_time setting of
new user on joining and instead we now use the twenty_four_hour_time
field of RealmUserDefault table for the same.

With some tweaks by tabbott to clarify the documentation.

zerver/lib/create_user.py

@@ -29,9 +29,7 @@ def copy_default_settings(
     #
     # Note that this function will do at least one save() on target_profile.
     for settings_name in UserBaseSettings.property_types:
-        if settings_name in ["default_language", "twenty_four_hour_time"] and isinstance(
-            settings_source, RealmUserDefault
-        ):
+        if settings_name == "default_language" and isinstance(settings_source, RealmUserDefault):
             continue
         value = getattr(settings_source, settings_name)
         setattr(target_profile, settings_name, value)
@@ -110,7 +108,6 @@ def create_user_profile(
         enter_sends=enter_sends,
         onboarding_steps=orjson.dumps([]).decode(),
         default_language=realm.default_language,
-        twenty_four_hour_time=realm.default_twenty_four_hour_time,
         delivery_email=email,
         **extra_kwargs,
     )