Remove functionality to merge duplicates

[Pennycook]

We previously disabled this functionally by setting merge_duplicates=False, but stopped short of removing the code.

Related issues

Arguably part of #36 -- all the code removed by this PR was previously unreachable and untested.

Proposed changes

• Remove merge_duplicates because it's always False.
• Remove _map_filename() because it does nothing when merge_duplicates=False.
• Remove FileInfo and compute_file_hash because they were only used by _map_filename.