Remove optional prefix field. #285

Got the message:
Unable to find css "option" with text /^\$NAME_PREFIX$/
["/home/congressforms/congress-forms/app/models/congress_member.rb:45:in `rescue in fill_out_form'", "/home/congressforms/congress-forms/app/models/congress_member.rb:39:in `fill_out_form'", "/home/congressforms/congress-forms/app/helpers/form_fill_handler.rb:15:in `block in create_thread'"]

Took out the field to see if the form can be submitted.

Also, change the selector of submit button to #submit id
Merge pull request #1261 from shwei/patch-8

Remove optional prefix field. #285