Merge CCoinsViewCache's GetOutputFor and AccessCoin

Summary:

They're doing the same thing now.

Partial backport of core PR10195:
https://github.com/bitcoin/bitcoin/pull/10195/commits/119e552f7ccd49c0137a3c6b4f94018a84d69620

This is a leftover from the backport that started with D342.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8254