chore: drop leftovers from the pre-JNI binary-execution approach

iperf3_binary_path_provider.dart held no code, only a comment saying the
extract-and-exec approach it belonged to had been replaced. path_provider was
a dependency of that same approach and no longer has a single usage; removing
it regenerates the desktop plugin registrants.

Also drops an unused import in main.dart.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/main.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import 'package:iperf3gui/data/services/iperf3_service.dart';
 import 'package:iperf3gui/ui/features/test_run/test_run_screen.dart';
 
 /// Application entry point.