Correct behavior when syncing with delete-destination-files

[adreed-msft]

The original code for delete-destination-files accidentally passed through all files when delete-destination-files was enabled.

This PR removes the accidental code and implements a test to validate that the override does not occur, further validating the two use cases this flag was intended to cover--

1. A blob previously written to via the dfs endpoint
2. A blob previously written to with another tool that uses different block ID lengths.