Use systemProxyForQuery() everywhere

For generic Unix, Qt automatically picks up http_proxy since
Qt 4.8 (commit 2ec929a2b). It also picks up e.g. https_proxy etc,
so it's better to rely on it.

On OS X we want to use the system proxy settings, that allow
also more advanced configuration like different proxies for
different services, passwords ...

Change-Id: I72de5e576e4d34d87f4c787bb597db0ca59a70e1
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>