#2533: Use different method to change Add Another text.

For some reason, I thought form_alter wouldn't work. Using preprocess broke it,
because the form uses the value of submit buttons as the op. Because I changed it in a
preprocess, the form api didn't know it had changed. It was trying to submit the whole form.
Delete old preprocess function replaced by form_alter.
Merge pull request #2557 from openscholar/2533-add_another_file_button

Use different method to change Add Another text.