It is a VERY bad idea to edit plugins directly. For one, you might break things, but for two, your changes will be reverted when the plugin updates. Better to hook the right event from functions.php and change things programmatically.
Sadly, as far as I know, there is not an option where you can choose “no thanks” to gravatar.
That said, if you inspect the element and find an id you can use that id to do a display:none;