JBMC: struct components cannot be methods

Cleanup of an unnecessary branch: Any such component would be a
pointer/reference, not an actual code-typed element.

Author: tautschnig

jbmc/src/java_bytecode/expr2java.cpp

@@ -126,7 +126,9 @@ std::string expr2javat::convert_struct(
 
   for(const auto &c : components)
   {
-    if(c.type().id() != ID_code)
+    DATA_INVARIANT(
+      c.type().id() != ID_code, "struct member must not be of code type");
+
     {
       std::string tmp=convert(*o_it);
       std::string sep;