to_datetime without time zone no longer converts DateTime to UTC

### Steps to reproduce

Updating some old code, I got bitten by this. In Timex 3.3.0, I could parse a timestamp in a known local time zone as follows:

```
Timex.parse!("2022-08-02T20:05", "{ISO:Extended}") |> Timex.to_datetime("America/Chicago") |> Time.to_datetime()
```

Now, in 3.7.8, the second `to_datetime` call does not change the time zone:

```
iex(4)> Timex.parse!("2022-08-02T20:05", "{ISO:Extended}") |> Timex.to_datetime("America/Chicago")
#DateTime<2022-08-02 20:05:00-05:00 CDT America/Chicago>
iex(5)> Timex.parse!("2022-08-02T20:05", "{ISO:Extended}") |> Timex.to_datetime("America/Chicago") |> Timex.to_datetime()
#DateTime<2022-08-02 20:05:00-05:00 CDT America/Chicago>
```

There is a simple workaround:

```
iex(6)> Timex.parse!("2022-08-02T20:05", "{ISO:Extended}") |> Timex.to_datetime("America/Chicago") |> Timex.to_datetime("Etc/UTC")
~U[2022-08-03 01:05:00Z]
```

Perhaps it was a deliberate change that `to_datetime` is a noop when passed an existing `DateTime` and no time zone argument, but the documentation does not make this clear. Either the old functionality of shifting to UTC needs to be restored, or the documentation needs to clearly cover this exception.