Delete sync file before stating backup
Delete sync file before stating backup

(cherry picked from commit 1afb2b4e0600ace2cd1eb8f33007a10b2105e88f)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend