login: simplify came_from validation

Even though only server-relative came_from URLs were ever generated,
the login controller allowed fully qualified URLs (URLs including
scheme and server). To avoid an open HTTP redirect (CWE-601), the code
included logic to prevent redirects to other servers. By requiring
server-relative URLs, this logic can simply be removed.

Note: SCRIPT_NAME is still not validated and it is thus possible to redirect
from one app to another on the same netloc.