qt: remove todo bug fix for old versions of Qt

Summary:
We require Qt >= 5.9.5, and this bug was fixed in 5.5

This is a backport of Core PR18769

Test Plan:
ninja && src/qt/bitcoin-qt
Settings -> Options -> Wallet -> Enable coin control features
Send -> Inputs... -> Tree mode

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8998