[qt] Simplifies boolean expression model && model->haveWatchOnly()

Summary:
The boolean expression model && model->haveWatchOnly() can be simplified to model->haveWatchOnly(), because if (!model || !model->getOptionsModel()) { return; } guards against a potential dereferenced null pointer.

Backport of Core PR 12074
https://github.com/bitcoin/bitcoin/pull/12074/files
Depends on D2875

Test Plan:
make check
bitcoin-qt # sanity checks on export txs

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2876