[wpilibc] Clean up include files (#2708)

Based on run of include-what-you-use.org to identify unused include files in various .h and .cpp files.

The changes mostly fall into 3 categories:
- Actually unused includes - copy-paste errors, not removing includes after cleaning up code, etc
- A too-broad include used where a more specific (and hopefully smaller) header will do
- Interface .h files including headers only needed by the .cpp implementation - moving from .h to .cpp
  will mean that code which uses the .h doesn't pay the price of processing the header file they don't need

hal/src/main/native/sim/CANAPI.cpp

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2018-2019 FIRST. All Rights Reserved.                        */
+/* Copyright (c) 2018-2020 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -7,16 +7,13 @@
 
 #include "hal/CANAPI.h"
 
-#include <atomic>
-#include <ctime>
-
 #include <wpi/DenseMap.h>
 
 #include "CANAPIInternal.h"
 #include "HALInitializer.h"
 #include "hal/CAN.h"
 #include "hal/Errors.h"
-#include "hal/HAL.h"
+#include "hal/HALBase.h"
 #include "hal/handles/UnlimitedHandleResource.h"
 
 using namespace hal;