Alternation of Log in Form in drupal 7
In order to delete the field in a form   you have to  print the form  either using dpm($form) or print_r($form) and there you will get the field of username use that field to unset in form_alter .

function hook_form_alter(&amp;$form, &amp;$form_state, $form_id){

  if (form_id=="Your_form_id"){

      unset($form['the_field_you_dont_want']);

        }
   }