[explorer] Remove the raw header

Summary: This is not user-friendly data to be displayed in an explorer, and for now it's not available 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/D15949