When you convert a web form that you've created in FormsCentral and download as a PDF, you have the option of not including a submit button. If you created a PDF form with the desktop version of Acrobat, it won't have a submit button unless you upload it to the FormsCentral service and enable it for submission.
Still, you can remove it in Acrobat by first saving a non-enabled version by selecting: File > Save A Copy
and opening the copy (it doens't happen automatically) and deleting the button.