[TASK] Disabling file deduplication by default.

File hashing is a pretty intensive task both for CPU and disk.
This feature seems not to be a must have and can produce unwanted side effects.
Very large file can take a lot of time to be hashed at the end of transfert, producing protential timeouts and user frustration.

Users can still enable file deduplication feature by setting `file_hash` to `md5`.

Signed-off-by: Jerome Jutteau <jerome@jutteau.fr>