Remove dead lowerCaseHashes rename logic in CleanAssetBundleFolder

## Context

`CleanAssetBundleFolder` in `Utils.cs` renames output bundle files from lowercase to proper-cased hashes using the `lowerCaseHashes` dictionary. This was needed when content hashes could have mixed case.

All Decentraland content hashes are bafkrei CIDs (base32-lowercase) — the rename is always a no-op.

## Cleanup

- Remove `lowerCaseHashes` field and `PopulateLowercaseMappings` from `AssetBundleConverter.cs`
- Remove the `lowerToUpperDictionary` parameter from `CleanAssetBundleFolder` in `Utils.cs`
- Update `IDirectory.CleanAssetBundleFolder` and its implementation to drop the dictionary parameter
- Remove the file rename logic (lines 546-551 in `Utils.cs`) — just delete the `.manifest` files

## References

Follow-up from PR #269.