'No alignment' option for images. Only tested on 3.0, though I imagine it would work on 3.1 fine. Apply with Object::add_extension('HtmlEditorField_Toolbar', 'HtmlEditorField_ToolbarExtension');

gistfile1.php

<?php
/**
 * Add 'no alignment' option when inserting images through 'Insert Media' form
 */
class HtmlEditorField_ToolbarExtension extends Extension {

	public function updateFieldsForImage(&$fields, $url, $file) {
		$className = $fields->fieldByName('CSSClass');
		$classes = $className->getSource();
		$classes = array_merge(array('' => _t('HtmlEditorField.CSSCLASSNONE', 'No alignment.')), $classes);
		$className->setSource($classes);
		return $fields;
	}

}