Cleanup resources of ClusterTask

This commit cleans up the unused functions for ClusterTask. It aims to keep the hygiene of the codebase
and gets the repo ready for swapping the storage version to v1.

Author: JeromeJu

pkg/reconciler/taskrun/resources/taskspec.go

@@ -32,9 +32,6 @@ type GetTask func(context.Context, string) (v1beta1.TaskObject, *v1beta1.ConfigS
 // GetTaskRun is a function used to retrieve TaskRuns
 type GetTaskRun func(string) (*v1beta1.TaskRun, error)
 
-// GetClusterTask is a function that will retrieve the Task from name and namespace.
-type GetClusterTask func(name string) (v1beta1.TaskObject, error)
-
 // GetTaskData will retrieve the Task metadata and Spec associated with the
 // provided TaskRun. This can come from a reference Task or from the TaskRun's
 // metadata and embedded TaskSpec.