Scaffolding blocks, when placed below another block, can break it.

Simply place a scaffolding, and hold crouch and jump while placing downward. You will travel upward, once your head makes contact with the block above, the next scaffolding will overwrite the block above that.

In this case bedrock.

Then you can clear the scaffolding and put a placeholder block just below the remaining bedrock, and use the same method to clear it.