don't cleanup cdrom files in apt-get update

Regression from merging 801745284905e7962aa77a9f37a6b4e7fcdc19d0 and
b0f4b486e6850c5f98520ccf19da71d0ed748ae4. While fine by itself, merged
the part fixing the filename is skipped if a cdrom source is
encountered, so that our list-cleanup removes what seems to be orphaned
files.

Closes: 765458
releasing package apt version 1.0.9.2ubuntu2