Remove unused proxyLock member from SuperVdsmProxy

The usage was removed in 812fe75

Change-Id: Ie86d50ab1ad41dbba34c051e00b0863b2b1067a8
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35445
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>