transfers: delete ACH file when a transfer is deleted

[adamdecaf]

We should delete the file to clean out space for other files.

Issue: https://github.com/moov-io/api/issues/56

[adamdecaf]

I think this fixes a bug where multiple transfers would have been created with the same ID.

[codecov-io]

Codecov Report

Merging #39 into master will decrease coverage by 3.68%.
The diff coverage is 41.07%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   38.01%   34.32%   -3.69%     
==========================================
  Files          14       14              
  Lines        1710     2214     +504     
==========================================
+ Hits          650      760     +110     
- Misses        904     1293     +389     
- Partials      156      161       +5

Impacted Files	Coverage Δ
database.go	36.66% <ø> (-3.34%)	⬇️
transfers.go	29.37% <35.13%> (-4.64%)	⬇️
pkg/achclient/client.go	59.09% <50%> (-4.43%)	⬇️
pkg/achclient/files.go	18.18% <57.14%> (+1.96%)	⬆️
microDeposits.go	42.24% <0%> (-6.68%)	⬇️
common.go	68.42% <0%> (-5.43%)	⬇️
depositories.go	35.61% <0%> (-5.2%)	⬇️
events.go	37.71% <0%> (-4.04%)	⬇️
originators.go	31.93% <0%> (-3.78%)	⬇️
... and 8 more

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