Fix snapshot download command error with --download-dir option in client

[dlachaume]

Content

This PR includes a fix related to an error mentioned in #979 issue.
db_pathdir value was always empty and was an unwanted duplicate of unpack_dir as explained in the issue.

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Comments

MithrilClientSnapshotService config has been removed as it's no longer used.
All tests passed.

Issue(s)

Closes #979

[jpraynaud]

LGTM 👍