AuditTechnicalMetadataFileList: only consider preserved files, touchups for recent data model changes

[jmartin-sul]

Why was this change made? 🤔

• since technical-metadata-service's database only tracks preserved files, filter non-preserved files from the list we send to techMD service for auditing
• remove a straggler usage of Dro, since we've gotten rid of that model (follow on from Refactors AuditTechnicalMetadataFileList to use RepositoryObjects. #5013)
• comment updates for both of the above changes

part of sul-dlss/technical-metadata-service#515

How was this change tested? 🤨

• re-ran the full report on QA. got 142 results instead of the 369 returned before this change.
• ran the techMD rake task techmd:generate_for_moab for a druid in the new result list, and saw it get remediated (job got queued and executed in techMD service, audit detected no error when re-run after that).
• confirmed that non-preserved file names were no longer fed to techMD's audit endpoint. e.g. for druid:wy216td0310 on QA, i originally got the result druid:wy216td0310: found technical-metadata-service database; inconsistencies with v1 cocina: {"missing_filenames"=>["example.jp2"]}. but after this change, the non-preserved example.jp2 was no longer sent to techMD, and the audit then detected no errors for that druid (before this change logs showed auditing {:druid=>"druid:wy216td0310", :file_list=>[{:filename=>"example.tiff", :md5=>"7c35bef13c1fdc201ff0c93a42085bc8"}, {:filename=>"example.jp2", :md5=>"8ff299eda08d7c506273840d52a03bf3"}]}, and after this change they showed auditing {:druid=>"druid:wy216td0310", :preserved_file_list=>[{:filename=>"example.tiff", :md5=>"7c35bef13c1fdc201ff0c93a42085bc8"}]}, and logged a response like druid:wy216td0310: audited technical-metadata-service; response status: 200; response body: {"missing_filenames":[],"unexpected_filenames":[],"mismatched_checksum_file_infos":[]}).
• confirmed that percent complete messages still show up in the logs

⚡ ⚠ If this change has cross service impact, including data writes to shared file systems, run integration tests and/or test in [stage|qa] environment, in addition to specs. ⚡