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.

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MFC after: 2 days