Drop doxygen link to string_constraintt

Doxygen gives warning on this link command, because it is not able to
resolve it.

Author: romainbrenguier

src/solvers/refinement/string_constraint.h

@@ -88,8 +88,7 @@ std::string from_expr(
   const irep_idt &identifier,
   const string_constraintt &expr);
 
-/// Checks the data invariant for \link string_constraintt
-/// \related string_constraintt
+/// Checks the data invariant for string_constraintt
 /// \param [in] expr: the string constraint to check
 /// \return whether the constraint satisfies the invariant
 bool is_valid_string_constraint(