[lldb][DWARFIndex][NFC] Remove DWARFIndex::IterationActionAdaptor

Now that all `DWARFIndex` APIs have been converted to use
`IterationAction`, the `IterationActionAdaptor` is unused.

Author: Michael137

lldb/source/Plugins/SymbolFile/DWARF/DWARFIndex.h

@@ -145,25 +145,6 @@ class DWARFIndex {
   IterationAction ProcessNamespaceDieMatchParents(
       const CompilerDeclContext &parent_decl_ctx, DWARFDIE die,
       llvm::function_ref<IterationAction(DWARFDIE die)> callback);
-
-  /// Helper to convert callbacks that return an \c IterationAction
-  /// to a callback that returns a \c bool, where \c true indicates
-  /// we should continue iterating. This will be used to incrementally
-  /// migrate the callbacks to return an \c IterationAction.
-  ///
-  /// FIXME: remove once all callbacks in the DWARFIndex APIs return
-  /// IterationAction.
-  struct IterationActionAdaptor {
-    IterationActionAdaptor(
-        llvm::function_ref<IterationAction(DWARFDIE die)> callback)
-        : m_callback_ref(callback) {}
-
-    bool operator()(DWARFDIE die) {
-      return m_callback_ref(std::move(die)) == IterationAction::Continue;
-    }
-
-    llvm::function_ref<IterationAction(DWARFDIE die)> m_callback_ref;
-  };
 };
 } // namespace dwarf
 } // namespace lldb_private::plugin