extensionDownloader: Stop setting proxy resolver explicitly

extensionDownloader: Stop setting proxy resolver explicitly

Quoting the documentation:

  In libsoup 2.44 and later, you can set the session's “proxy-resolver”
  property to the resolver returned by g_proxy_resolver_get_default() to
  get the same effect. Note that for "plain" SoupSessions (ie, not
  SoupSessionAsync or SoupSessionSync), this is done for you automatically.

libsoup 2.44 was released in 2013, so we can safely assume that Soup is
new enough to handle this for us.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1940>