Superfluous user_access check prevents programmatic signups

A user_access check for the "sign up for content" permission exists inside of signup_sign_up_user() and is unnecessary, since access checking is a appropriately used in _signup_current_user_signup() to prevent the signup button from being displayed to the user if she lacks that permission.

The access check in signup_sign_up_user() disrupts functionality in a forthcoming signup companion module, which programmatically signs up users who are not allowed to do so directly.

This patch removes the unnecessary access check. I tested by enabling the signup form for display both in the node content as well as the signups tab, and granted a test user the "edit own signups" and "sign up for content" permissions. Both before and after this patch was applied, the signup form correctly disappeared when the user lacked the "sign up for content" permission.

Thanks!