Remove ObjLookupCache.objptr

It was unused

src/gc/heap.cpp

@@ -693,7 +693,7 @@ GCAllocation* LargeArena::allocationFrom(void* ptr) {
 
 void LargeArena::prepareForCollection() {
     for (LargeObj* lo = head; lo; lo = lo->next) {
-        lookup.push_back(ObjLookupCache(lo, &lo->data[0], lo->size));
+        lookup.push_back(ObjLookupCache(&lo->data[0], lo->size));
     }
     std::sort(lookup.begin(), lookup.end(),
               [](const ObjLookupCache& lo1, const ObjLookupCache& lo2) { return lo1.data < lo2.data; });
@@ -904,7 +904,7 @@ GCAllocation* HugeArena::allocationFrom(void* ptr) {
 
 void HugeArena::prepareForCollection() {
     for (HugeObj* lo = head; lo; lo = lo->next) {
-        lookup.push_back(ObjLookupCache(lo, &lo->data[0], lo->size));
+        lookup.push_back(ObjLookupCache(&lo->data[0], lo->size));
     }
     std::sort(lookup.begin(), lookup.end(),
               [](const ObjLookupCache& lo1, const ObjLookupCache& lo2) { return lo1.data < lo2.data; });

src/gc/heap.h

@@ -365,11 +365,10 @@ private:
 };
 
 struct ObjLookupCache {
-    void* objptr;
     void* data;
     size_t size;
 
-    ObjLookupCache(void* objptr, void* data, size_t size) : objptr(objptr), data(data), size(size) {}
+    ObjLookupCache(void* data, size_t size) : data(data), size(size) {}
 };