Clarify the resource remove logic in runtime API

If the resource in the delete call does not exist, the runtime should
not return an error. This eliminates the need for kubelet to define a
resource "not found" error that every runtime has to return.

pkg/kubelet/api/v1alpha1/runtime/api.proto

@@ -16,6 +16,7 @@ service RuntimeService {
     rpc StopPodSandbox(StopPodSandboxRequest) returns (StopPodSandboxResponse) {}
     // DeletePodSandbox deletes the sandbox. If there are any running containers in the
     // sandbox, they should be force deleted.
+    // It should return success if the sandbox has already been deleted.
     rpc DeletePodSandbox(DeletePodSandboxRequest) returns (DeletePodSandboxResponse) {}
     // PodSandboxStatus returns the Status of the PodSandbox.
     rpc PodSandboxStatus(PodSandboxStatusRequest) returns (PodSandboxStatusResponse) {}
@@ -30,6 +31,7 @@ service RuntimeService {
     rpc StopContainer(StopContainerRequest) returns (StopContainerResponse) {}
     // RemoveContainer removes the container. If the container is running, the container
     // should be force removed.
+    // It should return success if the container has already been removed.
     rpc RemoveContainer(RemoveContainerRequest) returns (RemoveContainerResponse) {}
     // ListContainers lists all containers by filters.
     rpc ListContainers(ListContainersRequest) returns (ListContainersResponse) {}
@@ -49,6 +51,7 @@ service ImageService {
     // PullImage pulls a image with authentication config.
     rpc PullImage(PullImageRequest) returns (PullImageResponse) {}
     // RemoveImage removes the image.
+    // It should return success if the image has already been removed.
     rpc RemoveImage(RemoveImageRequest) returns (RemoveImageResponse) {}
 }