release: Include -dbg files in dvd image

In 2016, commit 8834318685bc reworked the code for excluding -dbg
files from install media, and in the process accidentally broke it
for dvd images ('grep -v' should have been 'grep -vE').

FreeBSD Update builds later began to depend on this, and in any case
since DVD images are intended as "include everything" images there's
no point excluding those; so remove the (broken) filtering from DVD
image builds.

Approved by: re (implicit, release build fixes)
Sponsored by: Amazon
MFC after: 2 days

(cherry picked from commit cdfd0600dc8882f0a0d0e6d9a1cdcf926edba6d6)
(cherry picked from commit ba7844e4c91505e397396ab981a0032699b0cd70)