wallet: Make WalletInitInterface and DummyWalletInit private

Summary: Backport 1/2 of core PR12836 (commit 39bc2fa)

Test Plan:

make check
bitcoin-qt --help

Ensure that bitcoind and bitcoin-qt can execute

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2273