samples: hci_uart_3wire: remove USB related code

[jfischer-no]

There is no advantage to using the USB CDC ACM UART in this sample.
chery-picked from #80985

[Thalley]

The hci_ samples are usually not really used as samples, but rather applications.

This PR removes a feature from one of these, and while the author claims that

There is no advantage to using the USB CDC ACM UART in this sample.

This may break things for anyone using that feature.

I don't think we really have any deprecation process for "samples", but perhaps we should at least mention this in the migration guide or release notes?

[cvinayak]

The hci_ samples are usually not really used as samples, but rather applications.

This PR removes a feature from one of these, and while the author claims that

There is no advantage to using the USB CDC ACM UART in this sample.

This may break things for anyone using that feature.

I don't think we really have any deprecation process for "samples", but perhaps we should at least mention this in the migration guide or release notes?

The sample is originally a copy-paste of the hci_uart sample and its readme.rst file. USB use in 3-wire UART sample is redundant and outside the scope of the sample.