mtree: Remove stray wi directory

Approved by: re (cperciva)
Reviewed by: imp, emaste
Fixes: a21def4d568f ("pccard: Remove wi(4) driver")
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D53264

(cherry picked from commit f942a7465469008f7538436a70f15a44cabecbb0)
(cherry picked from commit 0b45751456aa3411deef96b9f245359005d5ebd2)