Kill ModifyCoin

Summary:
This is the last step toward removing the per tx UTXO API and replace it with a per output API. This new API will let us upgrade the storage of the UTXO set on a per output basis.

This is based of work from Pieter Wuille and a continuation of D342 .

Depends on D470

Test Plan:
Updated tests

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D471