Remove unused async imports (#91)

Since this version doesn't support Dart pre 2.0, these imports are unnecessary.
diff --git a/lib/discovery.dart b/lib/discovery.dart
index a2f53c0..a72bb12 100644
--- a/lib/discovery.dart
+++ b/lib/discovery.dart
@@ -5,7 +5,6 @@
 @Deprecated("Use the package_config.json based API")
 library package_config.discovery;
 
-import "dart:async";
 import "dart:io";
 import "dart:typed_data" show Uint8List;