make zones_map private

I noticed there is no documentation for ActiveSupport::TimeZone.zones_map. I was using this method before to get an array of all the names of time zones:
```
ActiveSupport::TimeZones.zones_map(&:name)
```
Now zones_map seems to return a ThreadSafe::Cache instead of something I can map and call name on. So I switched to using .all:
```
ActiveSupport::TimeZones.all.map(&:name)
```

It seems like maybe zones_map wasn't something intended to be used in the public interface at all, or at least not anymore. If it is, can we write some documentation for it?