Spelling and grammar fixes for AI Translate

Fixing some spelling and grammar issues that where noted in Dries notes. Also removing redundant warning message about incomplete AI settings on the "User interface translation" form (/admin/config/regional/translate).

With current module when you do not set the default provider + model you get an error message and a warning message about the same thing. I am removing the warning message from ai_translate.module as the function call _ai_translate_check_default_provider_and_model() already provides an error message with the same message.

Before:

  if (!_ai_translate_check_default_provider_and_model()) {
    $message = t('Please select a default provider and model for translation via the <a href="@ai_settings">AI settings</a> page.', [
      '@ai_settings' => Url::fromRoute('ai.settings_form')->toString(),
    ]);
    \Drupal::messenger()->addWarning($message);
  }

After:

  if (!_ai_translate_check_default_provider_and_model()) {
    return;
  }