Remove unnecessary calls to ithread_exit

The ithread_exit wrapper function behaves inconsistently across
platforms; on windows, the argument is a thread handle, denoting
the thread that will be exited. On pthreads, ithread_exit only
terminates the current thread, not any other thread.

These both calls are at the end of each thread function (and the one
with a non-null parameter refers to the handle of the thread itself),
so the thread will exit immediately anyway.

Change-Id: I21e43c56874729452bd80b28ff351e3c5d3f087e

test/decoder/main.c

@@ -1612,9 +1612,6 @@ WORD32 display_thread(void *pv_ctx)
     }
     ps_app_ctx->disp_deinit(ps_app_ctx->pv_disp_ctx);
 
-    /* destroy the display thread */
-    ithread_exit(ps_app_ctx->display_thread_handle);
-
     return 0;
 }