xmlrpc: remove useless 'serverPort' attribute

The BindingXMLRPC class used to have one
'serverPort' attribute, which was used only on the migration
path.

Since commit 3be9ad1f278 was merged some time ago, this
attribute is unused and could be removed.

This patch does that.

Change-Id: I120754b2c934da3393e5ea4323225c14e25b2087
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35271
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: mooli tayer <mtayer@redhat.com>