Remove unreachable statement

nil_typet is deprecated, so this is a trivial fix to get rid of this use.

Author: tautschnig

jbmc/src/java_bytecode/java_types.cpp

@@ -199,7 +199,8 @@ typet java_type_from_char(char t)
   case 'd': return java_double_type();
   case 'z': return java_boolean_type();
   case 'a': return java_reference_type(void_typet());
-  default: UNREACHABLE; return nil_typet();
+  default:
+    UNREACHABLE;
   }
 }