Revert "src: keep main-thread Isolate attached to platform during Dis…

…pose"

This reverts commit e460f8c.

It is no longer necessary after the previous commit, and restores
consistency of the call order between the main thread code,
the other call sites, and the documentation.

Refs: nodejs#31795

src/node_main_instance.cc

@@ -102,12 +102,8 @@ NodeMainInstance::~NodeMainInstance() {
   if (!owns_isolate_) {
     return;
   }
-  // TODO(addaleax): Reverse the order of these calls. The fact that we first
-  // dispose the Isolate is a temporary workaround for
-  // https://github.com/nodejs/node/issues/31752 -- V8 should not be posting
-  // platform tasks during Dispose(), but it does in some WASM edge cases.
-  isolate_->Dispose();
   platform_->UnregisterIsolate(isolate_);
+  isolate_->Dispose();
 }
 
 int NodeMainInstance::Run() {