Removing admin bar from wordpress dashboard

I’m using wordpress multiste 3.3.1 I’m not gonna update it in the future. So I disabled all upgrade functions.

I want to remove the wordpress admin bar from both frontend as well as dashboard.

I can remove it from frontend using this code.

add_action( 'init', 'disable_admin_bar', 1 );
function disable_admin_bar() {
    add_filter( 'show_admin_bar', '__return_false' );
}

But i couldn’t find any solution to remove it from dashboard.

I don’t want to use css solution to hide admin bar and I’m ready to edit the core files to remove it

Can anyone help me to remove it completely?. Thanks