archival: remove prefix argument from archivers
When the archivers work on the full we can reuse the same archiver
with different prefixes (for different subrepositories).
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'