mtree: Remove stray an (Aironet) directory

Approved by: re (cperciva)
Reviewed by: imp
Fixes: 663b174b5b53 ("an: Remove driver")
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D53265

(cherry picked from commit f7ccf051016fe3989ef6fbf10adea3ce4131f91b)
(cherry picked from commit 3281756a24e24da3cb0f6a41cecfdf6c74a74532)