dav: Drop user from URI as a workaround for Nextcloud bug

Currently, it is not possible to connect to some Nextcloud servers when a username is part of URI. So this also affects volumes configured over GOA. The main problem seems to be that Nextcloud doesn't send the WWW-Authenticate header as a part of 401 response in certain cases (e.g. when 2FA is enabled). A recent libsoup change leads to the situation that DAV backend doesn't have a chance to specify a password as the authenticate signal is not called at all. This is because libsoup tries to authenticate with an empty password first. Let's drop the username from URI to avoid the password-less login attempt so the backend has at least one chance to try some password in this case...

Fixes: #617 (closed)