docs/arduino-nano-33-ble.rst: Connecting USB TTL cable information removed

Initially, Arudino did not enable USB CDC_ACM interface,
due to which once flashed, the user was forced to connect
an external USB TTL cable to connect the board.

This limitation is now resolved; hence, connecting USB
TTL cable information is removed.

Signed-off-by: Gururaj Shetty <gururaj.shetty@huawei.com>