Merge #13891: [RPC] Remove getinfo deprecation warning

Summary:
b2f23c41538eaadd71c373ada75dd3a982eeb8bf [RPC] Remove getinfo deprecation warning (John Newbery)

Pull request description:

`getinfo` was removed in V0.16. A removal warning message was left in place to tell users that the method had been removed. We can remove that entirely in V0.18.

Tree-SHA512: bf93fbcf57a9be480438dcbdcab2dfd69ce277218b10628776975b093b3ffd2caa1751e0fb4cb0245443c81465693e2b8750e96d3e38632a78bae5ffa04f9212

Backport of Core PR13891

Test Plan:

ninja
bitcoind
bitcoin-cli getinfo   # Method not found

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5727