cli/command/container: diff: remove redundant validation and cleanup

client.ContainerDiff already validates the given container name/ID, and
produces an error when empty, so we don't have to check for this;
https://github.com/moby/moby/blob/abba330bbfe10765822b59bb68af99db439736ba/client/container_diff.go#L13-L16

While updating, also;

- remove the diffOptions type, as there were no other options, and make
  the container name/ID a string argument.
- fix camelCase nameing of dockerCLI

Before this patch:

    docker diff ""
    Container name cannot be empty

With this patch:

    docker diff ""
    invalid container name or ID: value is empty

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/diff_test.go

@@ -77,17 +77,3 @@ func TestRunDiffClientError(t *testing.T) {
 	err := cmd.Execute()
 	assert.ErrorIs(t, err, clientError)
 }
-
-func TestRunDiffEmptyContainerError(t *testing.T) {
-	cli := test.NewFakeCli(&fakeClient{})
-
-	cmd := NewDiffCommand(cli)
-	cmd.SetOut(io.Discard)
-	cmd.SetErr(io.Discard)
-
-	containerID := ""
-	cmd.SetArgs([]string{containerID})
-
-	err := cmd.Execute()
-	assert.Error(t, err, "Container name cannot be empty")
-}