Remove initial check for invitation on form load

This fixes an annoyance (at least for me) when users load or visit the register page without an invitation code. This may not be how you'd like it, but I thought I'd submit it anyways. The check is still there for the actual invitation code on submit, so if they don't enter a code, then it will error like normal. This seems a bit more friendly to me instead of throwing the error in their face right away.