[backport#13966] Hide spendable label if private key is disabled

Summary:
https://github.com/bitcoin/bitcoin/pull/13966/commits/fe1ff5026bace9b43ac01c14a26ed0dd75ccc6f7

Partial backport of Core PR13966

Test Plan:

ninja check
./src/qt/bitcoind -testnet

create a watch-only wallet, import some random address, see that there is no spendable label

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7300