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/data/services/iperf3_binary_path_provider.dart

@@ -1,9 +0,0 @@
-// This file is kept as a stub — iperf3 is now compiled as a native
-// shared library (libiperf3jni.so) via JNI.
-// The old binary-path-provider pattern (extract ELF from assets → chmod → execve)
-// was replaced by the JNI approach after SELinux blocked binary execution.
-// See .hermes/plans/ for details on the migration.
-
-// The Iperf3Service now uses MethodChannel to communicate with Kotlin/JNI:
-//   Dart → MethodChannel → Kotlin → JNI → iperf3 (same process)
-// No file extraction, no chmod, no Process.start needed.