shell: remove Kconfig option CONFIG_UART_SHELL_ON_DEV_NAME

Follow up on commit bfd45e5b
("drivers: remove Kconfig option CONFIG_UART_CONSOLE_ON_DEV_NAME")
Now we can also remove Kconfig option CONFIG_UART_CONSOLE_ON_DEV_NAME
since all UART drivers are converted to devicetree and we can just use
DEVICE_DT_GET(DT_CHOSEN(zephyr_shell_uart)).

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>