Vita: fix thread detach. Remove leftover KillThread

src/thread/vita/SDL_systhread.c

@@ -107,13 +107,7 @@ void SDL_SYS_WaitThread(SDL_Thread *thread)
 
 void SDL_SYS_DetachThread(SDL_Thread *thread)
 {
-    /* !!! FIXME: is this correct? */
-    sceKernelDeleteThread(thread->handle);
-}
-
-void SDL_SYS_KillThread(SDL_Thread *thread)
-{
-    sceKernelDeleteThread(thread->handle);
+    /* Do nothing. */
 }
 
 int SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)