samples: doc: drop obsolete modemanager notice

As part of cleaning up usage of non-inclusive language in-tree,
I spotted an obsolete mention to a udev rule that ModemManager doesn't
use anymore. Drop it.
See commit 9639e52726854782508e35a75c6abf874bd9e608 in ModemManager
repo.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>