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