[vm] Remove dead regexp code.

TEST=ci
Change-Id: I976bcb7bf24f4a8134d5c1c5e20e8f98dc85cfbf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/499082
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/heap/heap.cc

@@ -543,11 +543,6 @@ void Heap::CollectOldSpaceGarbage(Thread* thread,
       }
     }
 
-    thread->isolate_group()->ForEachMutatorAtASafepoint([&](Thread* mutator) {
-      // Discard regexp backtracking stacks to further reduce memory usage.
-      mutator->CacheRegexpBacktrackStack(nullptr);
-    });
-
     RecordBeforeGC(type, reason);
     NOT_IN_PRODUCT(VMTagScope tagScope(thread, VMTag::kGCOldSpaceTagId));
     TIMELINE_FUNCTION_GC_DURATION(thread, "CollectOldGeneration");

runtime/vm/thread.h

@@ -1378,14 +1378,6 @@ class Thread : public ThreadState, public IntrusiveDListEntry<Thread> {
     return OFFSET_OF(Thread, thread_locals_);
   }
 
-  std::unique_ptr<VirtualMemory> TakeRegexpBacktrackStack() {
-    return std::move(regexp_backtracking_stack_cache_);
-  }
-
-  void CacheRegexpBacktrackStack(std::unique_ptr<VirtualMemory> stack) {
-    regexp_backtracking_stack_cache_ = std::move(stack);
-  }
-
  private:
   template <class T>
   T* AllocateReusableHandle();
@@ -1662,8 +1654,6 @@ class Thread : public ThreadState, public IntrusiveDListEntry<Thread> {
 
   MallocGrowableArray<ObjectPtr> pointers_to_verify_at_exit_;
 
-  std::unique_ptr<VirtualMemory> regexp_backtracking_stack_cache_ = nullptr;
-
   explicit Thread(bool is_vm_isolate);
 
   void StoreBufferRelease(