storage: drop an unused configurable sd_validate_timeout

Change-Id: Ic77b0a6f71f40fc0f3c0b51d04cc7ca77182a09d
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21060
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>