server: record ach_files_deleted metric

[adamdecaf]

Realized we didn't have this when working on moov-io/paygate#39

[codecov-io]

Codecov Report

Merging #408 into master will decrease coverage by 0.7%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #408      +/-   ##
=========================================
- Coverage    85.5%   84.8%   -0.71%     
=========================================
  Files         109     109              
  Lines        6817    8870    +2053     
=========================================
+ Hits         5829    7522    +1693     
- Misses        693    1053     +360     
  Partials      295     295

Impacted Files	Coverage Δ
server/files.go	1.29% <0%> (-0.31%)	⬇️
test/ach-adv-read/main.go	55.55% <0%> (-8.09%)	⬇️
test/ach-axt-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-ccd-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-iat-read/main.go	60% <0%> (-8%)	⬇️
test/ach-trc-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-trx-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-xck-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-web-read/main.go	65.71% <0%> (-7.62%)	⬇️
test/ach-shr-read/main.go	45.45% <0%> (-7.49%)	⬇️
... and 96 more

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