Step 1: Open functions.php file and add the filter code. It will restrict WordPress to add image dimension (width & height) while you upload next images via content editor or featured image box.
This code will not remove image dimension from the existing uploaded images.
Step 2: Add the CSS in your CSS file (style.css). This CSS will make the current all images to responsive. It does not matter if the images have hard coded dimension (width & height) or not.
Replace these class names ‘entry-content’, ‘comment-content’ with your absolute class names.