nanobsd: embedded: Remove stray comment

Remove an outdated comment about NanoBSD not being able to remove the
firstboot sentinel file.

It was fixed in cb51e2bc3db7 ("rc: Allow the removal of
firstboot_sentinel on read-only file systems").

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54977