Remove api-url-prefix default, and use the defaults from sign-addon instead

[Rob--W]

We should remove the default URL from api-url-prefix https://github.com/mozilla/web-ext/blob/9e23a4cccbfdb1a989d87d24387b26680c2e118e/src/program.js#L530 and rely on the default of sign-addon. Then we don't have to make changes such as #2121 to web-ext whenever the server updates.

After all, sign-addon is designed to work with a specific version of the AMO signing endpoint, there is no real point in overriding the URL from web-ext. The only potentially useful use case for including the default value is to show it the help message. That's not a good enough reason for keeping it.