Link custom apt sources to /etc/apt/sources.list.d/ directory

Previously, we appended the contents of our apt-source-append.list to the /etc/apt/sources.list file, which caused problems on multiple `vagrant provision` commands unless we also took over the entire list by using our apt-source-default.list file.

Because multiple .list files can be specified in the /etc/apt/sources.list.d/ directory, this is a much better option for us.
Remove unneeded apt-source-default.list file