vdsm: Remove dead code in MigrationSourceThread.run

Removed the mstate variable and the corresponding if as the
variable was not used otherwise and the condition always true.

Change-Id: Ie507413f07eed5e8c0958b74ae2103d11dd1e0f3
Signed-off-by: Martin Betak <mbetak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18336
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>