This method will generate a notice if you try and remove a sort field that isn't set. This can happen if you call getSortFields, which will return an array like:

[0] => 'fieldName asc',
[1] => 'fieldName desc

But if you try and call:

<?php
$object->removeSortField('fieldName asc');
?>

You will get a notice. Instead, you need to truncate the sort order (' asc', ' desc') part of it. Something like:

<?php
$fieldName = trim(str_replace(array('asc', 'desc')), '', $fieldName);
?>