arduino-nano-33-ble.rst: Remove section on installing bossac

This is no longer needed as we switched to a setup where we build bossac
with bitbake and use the internal version for flashing.

The result is a streamlined flashing experience where no extra
installation of the Arduino IDE and search for the PATH is needed
anymore.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>