Remove TaskExecutor and TaskScheduler

- As all execution and scheduling is now in reactor,
  remove all traces of these spring interfaces.
- Fixes #800

spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java

@@ -36,10 +36,6 @@ public class TimerSmokeTests {
 	private StateMachine<String, String> buildMachine() throws Exception {
 		StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();
 
-		builder.configureConfiguration()
-			.withConfiguration()
-				.taskExecutor(taskExecutor);
-
 		builder.configureStates()
 			.withStates()
 				.initial("initial")
@@ -61,10 +57,6 @@ public class TimerSmokeTests {
 	private StateMachine<String, String> buildMachine2() throws Exception {
 		StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();
 
-		builder.configureConfiguration()
-			.withConfiguration()
-				.taskExecutor(taskExecutor);
-
 		builder.configureStates()
 			.withStates()
 				.initial("initial").end("end").and()