Sorry, forgot that the login is not styled directly by the theme.
If you do not have one, you really should be creating a Child Theme for any changes, as when the main theme updates, all the changes you may have made to the main theme will be lost.
Once you have made a child theme, the following code goes in its functions.php
file:
function remove_login_logo_header_link() { ?>
<style type="text/css">
.login h1 a {
display: none;
}
</style>
<?php }
add_action( 'login_enqueue_scripts', 'remove_login_logo_header_link' );
This is an adaptation of this: Customizing the Login Form : Change the Login Logo