[explorer] Don't show the block nonce

Summary: This is not a very useful info and it removes the need for the BlockDetails object completely, which is absent from the in-node chronik.

Test Plan:

cargo run

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15946