in Lightroom for Desktop when you remove a photo it will ask to either remove from Lightroom or delete from disk. Lightroom mobile does not delete from disk/ device and absolutely needs to have this feature.
If you take a picture using the Lighroom mobile camera it creates and saves a file (DNG or JPEG) on your device as well as import into Lightroom mobile collection.
If you try to remove the photo from Lightroom mobile it removes the photo from Lightroom mobile but there is no option to delete that photo from the device.
You have to manually navigate to the folder on your device find the corresponding file by file name and delete it. This is very difficult to manage.