[REQ][Elm] Remove `DateTime` in favour of `Posix`

Created by: andys8

Is your feature request related to a problem? Please describe.

Generated code is obfuscating Posix with a new name DateTime.

Describe the solution you'd like

DateTime is an alias for Time.Posix. Since Time.Posix is now being used everywhere in Elm applications, it could make sense to remove the type alias for clarification. I think this is a left-over from the code base before Time.Posix was introduced.