remove taskrun from default prune resource
remove taskrun from default prune resource

this removes taskrun from default prune resource, now only
pipelinerun is default prune resource.
reason: taskruns associated with pipelineruns are by default
deleted, and if a taskrun is not associated to any pipelinerun
then user can explicitly add it to the prune resources.

Signed-off-by: Pradeep Kumar <89f9c159ede6c6ccc410acc84b2d41fcb248aa60@redhat.com>

tektoncd/operator,tektoncd/operator,tektoncd/operator