Don't poison Node48 reverseIndex

Follows the reasoning of "Remove redundant version update": a write can
never lower the logical max version of a key range, so there's no read that
didn't pass before that should pass after a write. The check stays exact
reading the physical childMaxVersion and reverseIndex of unused slots, so
scan16's whole-page reads of them are well-defined and intended. The client
requests only produced valgrind false positives. Keep the initialization,
drop the poisoning.

ConflictSet.cpp

@@ -571,11 +571,7 @@ inline void Node48::copyChildrenAndKeyFrom(const Node16 &other) {
   }
   // The reverseIndex bytes beyond numChildren are initialized so that the
   // SIMD loads of a full 16-byte page in scan16 are well-defined (reading
-  // indeterminate bytes is UB in C++). valgrind then treats them as undefined
-  // so that it can still flag any read that relies on them.
-  VALGRIND_MAKE_MEM_UNDEFINED(reverseIndex + numChildren,
-                              sizeof(reverseIndex) -
-                                  numChildren * sizeof(reverseIndex[0]));
+  // indeterminate bytes is UB in C++).
 }
 
 inline void Node48::copyChildrenAndKeyFrom(const Node48 &other) {
@@ -626,11 +622,7 @@ inline void Node48::copyChildrenAndKeyFrom(const Node256 &other) {
   memcpy(partialKey(), &other + 1, partialKeyLen);
   // The reverseIndex bytes beyond numChildren are initialized so that the
   // SIMD loads of a full 16-byte page in scan16 are well-defined (reading
-  // indeterminate bytes is UB in C++). valgrind then treats them as undefined
-  // so that it can still flag any read that relies on them.
-  VALGRIND_MAKE_MEM_UNDEFINED(reverseIndex + numChildren,
-                              sizeof(reverseIndex) -
-                                  numChildren * sizeof(reverseIndex[0]));
+  // indeterminate bytes is UB in C++).
 }
 
 inline void Node256::copyChildrenAndKeyFrom(const Node48 &other) {
@@ -2024,8 +2016,6 @@ Node *erase(Node *self, WriteContext *writeContext, bool logical) {
     }
     VALGRIND_MAKE_MEM_UNDEFINED(parent48->children + lastChildrenIndex,
                                 sizeof(parent48->children[0]));
-    VALGRIND_MAKE_MEM_UNDEFINED(parent48->reverseIndex + lastChildrenIndex,
-                                sizeof(parent48->reverseIndex[0]));
 
     if (needsDownsize(parent48)) {
       downsize(parent48, writeContext);