kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects. (#3154)

* kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects.

- See https://github.com/citra-emu/citra/pull/4710 for details.

src/core/hle/kernel/client_session.h

@@ -19,6 +19,9 @@ class Thread;
 
 class ClientSession final : public Object {
 public:
+    explicit ClientSession(KernelCore& kernel);
+    ~ClientSession() override;
+
     friend class ServerSession;
 
     std::string GetTypeName() const override {
@@ -34,12 +37,9 @@ public:
         return HANDLE_TYPE;
     }
 
-    ResultCode SendSyncRequest(SharedPtr<Thread> thread);
+    ResultCode SendSyncRequest(Thread* thread);
 
 private:
-    explicit ClientSession(KernelCore& kernel);
-    ~ClientSession() override;
-
     /// The parent session, which links to the server endpoint.
     std::shared_ptr<Session> parent;