1002495 - Fix related to org delete

An error related to modifying a new record was raised when deleting an
organization.
During an org delete the provider gets deleted which inturn tries to
delete the tasks associated to it. The task had dependent => :nullify
set to true, which made the task update the provider marked for
deletion.
This fix removes that  exclusive 'nullify' constraint since the default rails
strategy already does that. Also the fact that no where in the code
do we delete a task related to providers implies this updating provider
task id to null is ever necessary.