optimize request operation

TZImagePickerController/TZImagePickerController/TZImageRequestOperation.m

@@ -26,7 +26,6 @@
 
 - (void)start {
     self.executing = YES;
-    dispatch_async(dispatch_get_global_queue(0, 0), ^{
     [[TZImageManager manager] getPhotoWithAsset:self.asset completion:^(UIImage *photo, NSDictionary *info, BOOL isDegraded) {
         dispatch_async(dispatch_get_main_queue(), ^{
             if (!isDegraded) {
@@ -45,7 +44,6 @@
             }
         });
     } networkAccessAllowed:YES];
-    });
 }
 
 - (void)done {