engine: Removing harmfull code from persitance of task to DB

The following code which is removed is useless, because of json serializer will
not serialize parentParameters because of it is transient, but during setting them to null we can create a race
which will destroy a correct value, in case if command triggered couple parallel tasks

Change-Id: Ib64fd00c207eaab51585dca5634cfb15f3b83820
Signed-off-by: Michael Kublin <mkublin@redhat.com>