Remove mempool dependency on minRelayFee

Summary:
As per title, this dependency does not seem to actually be used
for anything meaningful.

Test Plan: make check

Reviewers: deadalnix, jasonbcox, sickpig, matiu, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T287

Differential Revision: https://reviews.bitcoinabc.org/D1173