Remove unused "refresh" arg

This fixes #5130 by removing this argument from del_repo. Modification
to the sources list should trigger an automatic apt-get update, and does
in all repo-related functions. This refresh argument doesn't exist in
the other functions (mod_repo, etc.) and should not be in del_repo
either.
Merge pull request #5632 from terminalmage/issue5130

Remove unused "refresh" arg