[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