Remove dead checkpoint test

Summary:
This is testing nothing. The checkpoint code is not even reached.

Because the block passed is not the genesis block, the code tries to fetch it's parent, which doesn't exist and as a result we cannot raach the checkpoint related code.

Test Plan:

ninja al check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6758