Fix unused variable and wrong warning workaround

I had been looking at the wrong line all along when attempting to fix:
```
core/os/memory.cpp:184:13: warning: unused variable 's' [-Wunused-variable]
                uint64_t *s = (uint64_t *)mem;
                          ^
```

core/os/memory.cpp

@@ -89,17 +89,8 @@ void *Memory::alloc_static(size_t p_bytes, bool p_pad_align) {
 	atomic_increment(&alloc_count);
 
 	if (prepad) {
-		// Clang 5 wrongly complains about 's' being unused,
-		// while it's used to modify 'mem'.
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunused-variable"
-#endif // __clang__
 		uint64_t *s = (uint64_t *)mem;
 		*s = p_bytes;
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif // __clang__
 
 		uint8_t *s8 = (uint8_t *)mem;
 
@@ -181,9 +172,9 @@ void Memory::free_static(void *p_ptr, bool p_pad_align) {
 
 	if (prepad) {
 		mem -= PAD_ALIGN;
-		uint64_t *s = (uint64_t *)mem;
 
 #ifdef DEBUG_ENABLED
+		uint64_t *s = (uint64_t *)mem;
 		atomic_sub(&mem_usage, *s);
 #endif