Remove dead fields from PerIsolateThreadData

R=mstarzinger@chromium.org

Change-Id: Iefd80d3365369dbf9bfb9832640d90963253e603
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1593082Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61152}

src/isolate.h

@@ -460,12 +460,12 @@ class Isolate final : private HiddenFactory {
         : isolate_(isolate),
           thread_id_(thread_id),
           stack_limit_(0),
-          thread_state_(nullptr),
+          thread_state_(nullptr)
 #if USE_SIMULATOR
-          simulator_(nullptr),
+          ,
+          simulator_(nullptr)
 #endif
-          next_(nullptr),
-          prev_(nullptr) {
+    {
     }
     ~PerIsolateThreadData();
     Isolate* isolate() const { return isolate_; }
@@ -492,9 +492,6 @@ class Isolate final : private HiddenFactory {
     Simulator* simulator_;
 #endif
 
-    PerIsolateThreadData* next_;
-    PerIsolateThreadData* prev_;
-
     friend class Isolate;
     friend class ThreadDataTable;
     friend class EntryStackItem;