Merge #11385: Remove some unused functions and methods

Summary:
46c9043 Remove some unused functions and methods (Pieter Wuille)

Pull request description:

In the case of CKey's destructor, it seems to have been an oversight in #8753 not to delete it. At this point, it results in the move constructors/assignment operators for CKey being deleted, which may have
a performance impact (requiring a pool allocation/copy/free, rather than just handing over the pointer from one CKey to another)

Tree-SHA512: 89715bafe3e0bea2c46fc92bc6a1010360a3fee2719f97b81ca379581003409b0876b50f992208a3c13c7f5b77f1866db09954e7d102f6a452fe5d7aed2044a1

Backport of Core PR11385
https://github.com/bitcoin/bitcoin/pull/11385

Test Plan:

make check
test_runner.py

Reviewers: jasonbcox, deadalnix, Fabien, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3044