remove unused batch indexing code

[jveitchmichaelis]

There's a lot of code that is no longer used, as we have moved to a more robust way of tracking where image crops came from when predicting over multiple images. This PR removes that so there's less confusion, and adds a test to check that windows are properly preserved via metadata.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.73%. Comparing base (d2851ec) to head (b36f8e0).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/deepforest/datasets/prediction.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1406      +/-   ##
==========================================
+ Coverage   86.61%   86.73%   +0.11%     
==========================================
  Files          26       28       +2     
  Lines        3736     4063     +327     
==========================================
+ Hits         3236     3524     +288     
- Misses        500      539      +39     

Flag	Coverage Δ
unittests	86.73% <83.33%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.