[core] remove unnecessary forward declarations of class ThreadNetif (#12278)

Removes unnecessary `class ThreadNetif;` forward declarations in
`meshcop.hpp`, `child_supervision.hpp`, and `jam_detector.hpp`.

src/core/meshcop/meshcop.hpp

@@ -55,9 +55,6 @@
 #include "meshcop/steering_data.hpp"
 
 namespace ot {
-
-class ThreadNetif;
-
 namespace MeshCoP {
 
 /**

src/core/thread/child_supervision.hpp

@@ -51,8 +51,6 @@
 
 namespace ot {
 
-class ThreadNetif;
-
 /**
  *
  * Child supervision feature provides a mechanism for parent

src/core/utils/jam_detector.hpp

@@ -47,9 +47,6 @@
 #include "common/timer.hpp"
 
 namespace ot {
-
-class ThreadNetif;
-
 namespace Utils {
 
 class JamDetector : public InstanceLocator, private NonCopyable