WordPress Filter to remove "height" and "width" attributes and values of image only.

remove-img-attr.php

<?php

add_filter( 'post_thumbnail_html', 'twr_remove_width_attribute', 10 );
add_filter( 'image_send_to_editor', 'twr_remove_width_attribute', 10 );
add_filter(	'the_content', 'twr_remove_width_attribute', 10);

function twr_remove_width_attribute($html) {
    // loop only through img tags
    if ( preg_match_all( '/<img[^>]+>/ims', $html, $matches ) ) {

        foreach ( $matches as $match ) {
            // Replace all occurences of width/height
            $clean = preg_replace('/(width|height)=["\'\d%\s]+/ims', "", $match );
            // Replace with result within html
            $html = str_replace( $match, $clean, $html );

        }
    }
    return $html;
}