Removing unnecessary Nullable code. In preparation for androidx.

See
itinance#691 (comment)

Author: Alistair Nelson

android/src/main/java/com/rnfs/RNFSManager.java

@@ -8,7 +8,7 @@
 import android.os.Environment;
 import android.os.StatFs;
 import android.provider.MediaStore;
-import android.support.annotation.Nullable;
+
 import android.util.Base64;
 import android.util.SparseArray;
 import android.media.MediaScannerConnection;
@@ -682,7 +682,7 @@ public void mkdir(String filepath, ReadableMap options, Promise promise) {
     }
   }
 
-  private void sendEvent(ReactContext reactContext, String eventName, @Nullable WritableMap params) {
+  private void sendEvent(ReactContext reactContext, String eventName, WritableMap params) {
     reactContext
             .getJSModule(RCTNativeAppEventEmitter.class)
             .emit(eventName, params);