Remove dead code; Clean up code quality per IDE suggestions

api/_routers/01-install_metadata.go

api/_routers/97-optional-access-token.go

api/_routers/97-require-access-token.go

api/custom/imports.go

api/custom/purge.go

api/r0/download.go

api/r0/identicon.go

api/r0/preview_url.go

api/r0/thumbnail.go

api/r0/upload_async.go

api/r0/upload_sync.go

api/router.go

api/unstable/info.go

api/unstable/local_copy.go

api/webserver.go

archival/entity_export.go

common/LEGACY_media_kinds.go

common/errorcodes.go

common/globals/LEGACY_singleflight_groups.go

controllers/LEGACY.txt