samples: hci_uart: drop device_get_binding()

Use DEVICE_DT_GET on the chosen node directly.

The only in-tree board using the Kconfig option is rv32m1_vega_ri5cy.
It already sets the chosen node in the application overlay, albeit
incorrectly, so just fix it there and remove the now-redundant Kconfig
setting. That in turn triggers other build errors for this board, but
they don't seem to be related to this sample.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>