Implement hook_form_workflow_transition_form_alter().
/**
* Implements hook_form_form_id_alter().
* Alter transition form to remove current state button.
*/
function MYMODULE_form_workflow_transition_form_alter(&$form, &$form_state, $form_id) {
// Get current state to remove current state button.
$current_sid = $form['workflow']['workflow_sid']['#default_value'];
unset($form['workflow']['workflow_sid']['#options'][$current_sid]);
// By default workflow button label became (Update workflow) if the options
// count equal one. So, We will use the state label name instead of
// (Update workflow).
// Check if option count equal one.
if (is_array($form['workflow']['workflow_sid']['#options']) && (count($form['workflow']['workflow_sid']['#options']) == 1)) {
$submit_label = current($form['workflow']['workflow_sid']['#options']);
$form['workflow']['actions']['submit']['#value'] = $submit_label;
}
}