[backport#15639 3/3] bitcoin-wallet tool: Drop libbitcoin_server.a dependency

Summary:
This ensures wallet code doesn't access node global state, avoiding bugs like
https://github.com/bitcoin/bitcoin/pull/15557#discussion_r267735431

https://github.com/bitcoin/bitcoin/pull/15639/commits/78a2fb55c97fbc26f7b74c5b1fb999a2aff8ce88

Test Plan:

ninja check-all [also with -DBUILD_BITCOIN_WALLET=OFF]

make check-recursive
./test/functional/test_runner.py [also with --disable-wallet]

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6795