migration: to_dst_file at that point is NULL

migration: to_dst_file at that point is NULL

We have just arrived as:

migration.c: qemu_migrate()
  ....
  s = migrate_init() <- puts it to NULL
  ....
  {tcp,unix}_start_outgoing_migration ->
     socket_outgoing_migration
        migration_channel_connect()
	   sets to_dst_file

if tls is enabled, we do another round through
migrate_channel_tls_connect(), but we only set it up if there is no
error.  So we don't need the assignation.  I am removing it to remove
in the follwing patches the knowledge about MigrationState in that two
files.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>