gui: move coin control OK button to the right

Summary:
BitcoinQT's Input selector is showing the OK button on the left side instead of the right side.
All the buttons in the app show on the right side.
The simplest solution is to just drop the sizePolicy property, as we don't use that on any other instances of QDialogButtonBox.

Backport of Core PR16113

Test Plan:

ninja
./src/qt/bitcoin-qt

In menu Settings > Options > Wallet, check Enable coin control features.
Back to the main window, on the Send tab, click on the Inputs buttons,
check that the dialog now has the OK button on the bottom right.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7797