Remove *_source.changes file generated by dpkg-buildpackage in *-source job [Closes: #67]

The recent commit to make sure quilt can't break the build (git
commit 63d6c47) made a switch from dpkg-source to
dpkg-buildpackage. dpkg-buildpackage generates a *_source.changes
file which isn't present when using dpkg-source. While this file
is nice it doesn't provide any real value for us, but what's more
important, it might break existing setups:

If all files are copied around from one Jenkins job to another
then in the final jenkins job the changes file from the binaries
job and the source.changes one end up together. If those are
passed to other tools then things might break (see github issue
67 as an example). So let's just remove the file after the
dpkg-buildpackage call to provide the used behaviour.