Remove minTxFee from wallet

Summary:
Currently, the wallet has a minTxFee which will override the minrelayfee
when calculating minimum fees. This leads to a situation where both must
be adjusted together in order to get the desired effect. -paytxfee should
be used instead, if overriding minrelayfee is desired.

Test Plan: make check && ./test/function/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1800