[chronik] Show the chronik options

Summary: Chronik is now stable, move the options out of the hidden category.

Test Plan:
Build with Chronik, then:

./src/bitcoind -help

Check the chronik options are displayed.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15880