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

@@ -89,8 +89,7 @@ exprt univ_within_bounds(const string_constraintt &axiom);
 /// \return rendered string
 std::string to_string(const string_constraintt &expr);
 
-/// Checks the data invariant for \link string_constraintt
-/// \related string_constraintt
+/// Checks the data invariant for string_constraintt
 /// \param [in] constraint: the string constraint to check
 /// \return whether the constraint satisfies the invariant
 bool is_valid_string_constraint(