remove unusued dependency

Author: jaredmixpanel

android/src/main/java/com/mixpanel/mixpanel_flutter/MixpanelFlutterPlugin.java

@@ -19,7 +19,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 import com.mixpanel.android.mpmetrics.MixpanelAPI;