[lldb] Remove Base::unique from NonNullSharedPtr (llvm#169130)

It seems like this fails on macOS with C++20

Author: keith

lldb/include/lldb/Utility/NonNullSharedPtr.h

@@ -56,7 +56,6 @@ template <typename T> class NonNullSharedPtr : private std::shared_ptr<T> {
   using Base::operator*;
   using Base::operator->;
   using Base::get;
-  using Base::unique;
   using Base::use_count;
   using Base::operator bool;