[TargetList] Delete the destructor

[vsk]

Summary

AFAICT, ~TargetList simply implements the default destructor, plus some
locking.

The history is murky, so I'm not sure why we do this locking. Perhaps,
at some point, it was possible to delete the same TargetList instance
from two different threads, setting up a race. If that were true, then
the locking would protect against the race.

Since TargetList is uniquely owned by Debugger (m_target_list), no such
race is possible today.

Testing: check-lldb

[JDevlieghere]

LGTM

lldb/include/lldb/Target/TargetList.h

public:
   // These two functions fill out the Broadcaster interface:
 
   static ConstString &GetStaticBroadcasterClass();
 
   ConstString &GetBroadcasterClass() const override {
     return GetStaticBroadcasterClass();
   }
 
-  ~TargetList() override;
-
   /// Create a new Target.
   ///
   /// Clients must use this function to create a Target. This allows
   /// a global list of targets to be maintained in a central location
   /// so signal handlers and other global functions can use it to
   /// locate an appropriate target to deliver asynchronous information
   /// to.
   ///

lldb/source/Target/TargetList.cpp

 // TargetList constructor
 TargetList::TargetList(Debugger &debugger)
     : Broadcaster(debugger.GetBroadcasterManager(),
                   TargetList::GetStaticBroadcasterClass().AsCString()),
       m_target_list(), m_target_list_mutex(), m_selected_target_idx(0) {
   CheckInWithManager();
 }
 
-// Destructor
-TargetList::~TargetList() {
-  std::lock_guard<std::recursive_mutex> guard(m_target_list_mutex);
-  m_target_list.clear();
-}
-
 Status TargetList::CreateTarget(Debugger &debugger,
                                 llvm::StringRef user_exe_path,
                                 llvm::StringRef triple_str,
                                 LoadDependentFiles load_dependent_files,
                                 const OptionGroupPlatform *platform_options,
                                 TargetSP &target_sp) {
   return CreateTargetInternal(debugger, user_exe_path, triple_str,
                               load_dependent_files, platform_options,