Fix dangling doxygen ref.

Author: Remi Delmas

src/goto-instrument/contracts/dynamic-frames/dfcc_instrument.h

@@ -285,8 +285,6 @@ class dfcc_instrumentt
 
   /// Instrument the \p lhs of an `ASSIGN lhs := rhs` instruction by
   /// adding an inclusion check of \p lhs in \p write_set.
-  /// If \ref is_dead_object_update returns a successful match, the matched
-  /// pointer expression is removed from \p write_set.
   /// If \p rhs is a `side_effect_expr(ID_allocate)`, the allocated pointer gets
   /// added to the \p write_set.
   /// \pre \p target points to an `ASSIGN` instruction.