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.cpp

@@ -313,6 +313,4 @@ SharedPtr<Process> KernelSystem::GetProcessById(u32 process_id) const {
 
     return *itr;
 }
-
-SharedPtr<Process> g_current_process;
 } // namespace Kernel