qt: Use parent-child relation to manage lifetime of OptionsModel object

Summary:
Both BitcoinApplication and OptionsModel classes are derived from the
QObject class, therefore a parent-child relation could be established to
manage the lifetime of an OptionsModel object.
This commit does not change behavior.

This is a backport of Core PR18424

Test Plan: ninja && src/bitcoin-qt

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9255