ObsoleteFiles: Remove if_rtwn.4 removal

if_rtwn.4 was previously removed, but actually it is correct because
the interfaces is called rtwn.

Fixes: 4262dbc579823
MFC after: 3 days
Reported by: bz
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49323

(cherry picked from commit 732c16884215c0117d5ee3df79ca79c21ad07e19)