kernel/thread: Move process thread initialization into process.cpp

This function isn't a general purpose function that should be exposed to
everything, given it's specific to initializing the main thread for a
Process instance.

Given that, it's a tad bit more sensible to place this within
process.cpp, which keeps it visible only to the code that actually needs
it.

src/core/hle/kernel/thread.h

@@ -455,17 +455,6 @@ private:
     ThreadActivity activity = ThreadActivity::Normal;
 };
 
-/**
- * Sets up the primary application thread
- * @param kernel The kernel instance to create the main thread under.
- * @param entry_point The address at which the thread should start execution
- * @param priority The priority to give the main thread
- * @param owner_process The parent process for the main thread
- * @return A shared pointer to the main thread
- */
-SharedPtr<Thread> SetupMainThread(KernelCore& kernel, VAddr entry_point, u32 priority,
-                                  Process& owner_process);
-
 /**
  * Gets the current thread
  */