Originally seen while investigating wordpress-mobile/WordPress-Android#9377

If an upload is happening, and the user deletes the image block containing the image being uploaded, we should cancel the upload, just like we do in onUndoMediaCheck() for Aztec.

To test:

1. start a new draft
2. insert an image block
3. choose an image from the device to start the upload
4. once the upload starts, tap on the block's trash icon
5. observe the image continues being uploaded, even when there's no longer a block where to put the image into.