Remove System::Object (#12051)

#### Problem

All uses of `System::Object` pools have now converted to
use `chip::ObjectPool` from `lib/support/Pool.h`.

#### Change overview

- Remove `System::Object`
- Fix some formerly transitive includes.

#### Testing

CI for build. No change to active code.

examples/shell/shell_common/cmd_send.cpp

@@ -18,6 +18,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include <lib/core/CHIPCore.h>
 #include <lib/shell/Engine.h>

src/inet/tests/TestSetupFaultInjectionPosix.cpp

@@ -32,9 +32,11 @@
 #include <lib/support/CHIPFaultInjection.h>
 #include <lib/support/CodeUtils.h>
 #include <lib/support/ScopedBuffer.h>
-#include <stdio.h>
 #include <system/SystemFaultInjection.h>
 
+#include <stdio.h>
+#include <unistd.h>
+
 struct RestartCallbackContext
 {
  int mArgc;

src/protocols/user_directed_commissioning/UserDirectedCommissioningClient.cpp

@@ -25,6 +25,8 @@
 
 #include "UserDirectedCommissioning.h"
 
+#include <unistd.h>
+
 namespace chip {
 namespace Protocols {
 namespace UserDirectedCommissioning {

src/system/BUILD.gn

@@ -139,8 +139,6 @@ static_library("system") {
  "SystemLayerImpl.h",
  "SystemMutex.cpp",
  "SystemMutex.h",
- "SystemObject.cpp",
- "SystemObject.h",
  "SystemPacketBuffer.cpp",
  "SystemPacketBuffer.h",
  "SystemStats.cpp",

src/system/SystemLayer.h

@@ -37,7 +37,6 @@
 #include <system/SystemClock.h>
 #include <system/SystemError.h>
 #include <system/SystemEvent.h>
-#include <system/SystemObject.h>
 #include <system/SystemTimer.h>
 
 #if CHIP_SYSTEM_CONFIG_USE_SOCKETS