Remove "Next" button in register to service dialog

[sander]

When registering to a transport manually (using the Services tab), the dialog will show a button called "Next" which in fact will do a Clean action. The current "Next" is really confusing, and a "Clean" button isn't really that necessary (people can click Cancel and try again or empty the fields manually). Besides that it's also not consistent with other dialogs like the register new account dialog as these also do not have such a button.

Therefore: simply remove this button.