hide button "SUBMIT" in currency block selector

Hiding the button from CSS or removing it would cause accessibility rule validation issues (SC 3.2.2: The submit button is missing). To avoid that, the correct way is to hide it visually:

In MYTHEME.theme:

<?php
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function MYTHEME_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'commerce_currency_resolver_select_form') {
    $form['submit']['#attributes']['class'][] = 'visually-hidden';
  }
}
?>

It could be hided with css, for example:

.topbar-currency-region .commerce-currency-resolver-select-form .button {
    display: none;
}