SoupSession: remove some dead code, fix proxy error messages

SoupSession: remove some dead code, fix proxy error messages

SoupMessageQueueItem's proxy_addr and proxy_uri fields were no longer
being used, but they were still there, and SoupSession was trying to
use item->proxy_uri to provide clarifying information on
SOUP_STATUS_CANT_RESOLVE_PROXY / SOUP_STATUS_CANT_CONNECT_PROXY.

Remove the unused fields, and fix SoupSession to get the proxy name
from the SoupConnection instead.

https://bugzilla.gnome.org/show_bug.cgi?id=680273