boards: arduino_nano_33_ble: drop BLE_EN_USB_CONSOLE

Drop the BOARD_ARDUINO_NANO_33_BLE_EN_USB_CONSOLE Kconfig option. If
applications need a USB console, they need to adjust `prj.conf` (or
overlays) enabling relevant settings.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>