1. Go to the Develop module.
2. Select the images you want to remove the masks.
3. Make sure the Sync button switch is turned on.
4. Click the Masking icon, or press the Shift+W keys.
5. On the bottom of the Masking section, click the Delete all masks button.
   
   
   

Another method of removing the Imagen mask:

1. Remove the mask for one image.
2. Select the rest of the photos you want to remove the masks from, while making sure you are viewing the photo without the mask.
3. Sync the Masking option.