[lldb] Remove unused ThreadPlanStack::GetStackOfKind (NFC)

[kastiglione]

Summary

This function isn't used.

[jingham]

The inverse change to this would be to have all the places where we ask questions about the PlanStacks in a ThreadPlanStack through this API. I put it in to add that abstraction layer, but I don't remember why I thought it might be useful, and I never found any actual motivation for it. So this can go.

lldb/include/lldb/Target/ThreadPlanStack.h

 // it is detached from a thread, all you can do is reattach it or delete it.
 class ThreadPlanStack {
   friend class lldb_private::Thread;
 
 public:
   ThreadPlanStack(const Thread &thread, bool make_empty = false);
   ~ThreadPlanStack() {}
 
-  enum StackKind { ePlans, eCompletedPlans, eDiscardedPlans };
-
   using PlanStack = std::vector<lldb::ThreadPlanSP>;
 
   void DumpThreadPlans(Stream &s, lldb::DescriptionLevel desc_level,
                        bool include_internal) const;
 
   size_t CheckpointCompletedPlans();
 
   void RestoreCompletedPlanCheckpoint(size_t checkpoint);
public:
 
   /// Clear the Thread* cache that each ThreadPlan contains.
   ///
   /// This is useful in situations like when a new Thread list is being
   /// generated.
   void ClearThreadCache();
 
 private:
-  const PlanStack &GetStackOfKind(ThreadPlanStack::StackKind kind) const;
-
   void PrintOneStack(Stream &s, llvm::StringRef stack_name,
                      const PlanStack &stack, lldb::DescriptionLevel desc_level,
                      bool include_internal) const;
 
   PlanStack m_plans;           ///< The stack of plans this thread is executing.
   PlanStack m_completed_plans; ///< Plans that have been completed by this
                                /// stop.  They get deleted when the thread
                                /// resumes.

lldb/source/Target/ThreadPlanStack.cpp

for (lldb::ThreadPlanSP thread_plan_sp : m_plans)
     thread_plan_sp->ClearThreadCache();
 }
 
 void ThreadPlanStack::WillResume() {
   m_completed_plans.clear();
   m_discarded_plans.clear();
 }
 
-const ThreadPlanStack::PlanStack &
-ThreadPlanStack::GetStackOfKind(ThreadPlanStack::StackKind kind) const {
-  switch (kind) {
-  case ePlans:
-    return m_plans;
-  case eCompletedPlans:
-    return m_completed_plans;
-  case eDiscardedPlans:
-    return m_discarded_plans;
-  }
-  llvm_unreachable("Invalid StackKind value");
-}
-
 void ThreadPlanStackMap::Update(ThreadList &current_threads,
                                 bool delete_missing,
                                 bool check_for_new) {
 
   // Now find all the new threads and add them to the map:
   if (check_for_new) {
     for (auto thread : current_threads.Threads()) {
       lldb::tid_t cur_tid = thread->GetID();