refactor: Remove unused COINBASE_FLAGS

Summary:

Commit d449772cf69c01932fc5d72c46054815d6300b3c stopped setting
COINBASE_FLAGS, and it looks like it hasn't been used since P2SH.
Update the help string to remove "flags", which is not specified in
BIP 22.

One occurence of flags in the help message was already removed in D8124

This is a backport of Core PR17519

Test Plan:
Verify that nothing is broken as a result of removing pushKV("flags"...
ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8275