Remove ::minRelayTxFee global

Summary:
The -incrementalrelayfee option was removed from Bitcoin ABC in v0.17.2

This is a partial backport of core#25648
https://github.com/bitcoin/bitcoin/pull/25648/commits/fa9cba7afb73c01bd2c8fefd662dfc80dd98c5e8

Depends on D15752

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15753