cleanup: remove startCommand function, only used once, and

unecessary abstraction.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)

integration-cli/docker_cli_attach_test.go

@@ -50,7 +50,7 @@ func TestAttachMultipleAndRestart(t *testing.T) {
 				t.Fatal(err)
 			}
 
-			if _, err := startCommand(c); err != nil {
+			if err := c.Start(); err != nil {
 				t.Fatal(err)
 			}