[record_use] Remove const_finder (#191702)

Removes `const_finder` now that all uses (including in g3) have been
migrated to `package:record_use`.

- Delete `engine/src/flutter/tools/const_finder/`
- Remove `const_finder` references in engine build configs (`BUILD.gn`,
`run_tests.py`, `pub_get_offline.py`, `pubspec.yaml`)
- Remove `const_finder.dart.snapshot` from `font_subset` bundle in
`engine/src/flutter/tools/font_subset/BUILD.gn`
- Remove `Artifact.constFinder` enum value and implementations from
`packages/flutter_tools/lib/src/artifacts.dart`
- Update doc comments referencing `const_finder` in
`packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart`

Note, the const-finder sources were dead code already because
const_finder was moved to the frontend_server in the Dart SDK at some
point.

Related Dart CL: https://dart-review.googlesource.com/c/sdk/+/541180