If you want to remove "Choose Currency" option from the cart (and so forcing the user to use just default currency), insert the following code in your /includes/hooks/navbarlinks.php file:
add_hook('ClientAreaSecondarySidebar', 1, function (MenuItem $secondarySidebar) {
if (!is_null($secondarySidebar->getChild('Choose Currency'))) {
$secondarySidebar->removeChild('Choose Currency');
}
});