Add new mju_threadpool API function, and delete old threading API.

PiperOrigin-RevId: 922838541
Change-Id: Id9f7e0fb298ffde61fcc49a802dc78971858ce51

src/engine/engine_memory.h

@@ -49,9 +49,6 @@ void mj__freeStack(mjData* d) __attribute__((noinline));
 
 #endif  // ADDRESS_SANITIZER
 
-// returns the number of bytes available on the stack
-MJAPI size_t mj_stackBytesAvailable(mjData* d);
-
 // allocate bytes on the stack
 MJAPI void* mj_stackAllocByte(mjData* d, size_t bytes, size_t alignment);