kernel/process: move current process to kernel instance

Two functional change:
QueryProcessMemory uses the process passed from handle instead current_process
Thread::Stop() uses TLS from owner_process instead of current_process

src/core/hle/kernel/process.h

@@ -197,6 +197,4 @@ private:
     friend class KernelSystem;
     KernelSystem& kernel;
 };
-
-extern SharedPtr<Process> g_current_process;
 } // namespace Kernel