LibSQL: Remove the now-unused ExecutionContext::result object

The INSERT and SELECT statements set up this object for any expression
evaluation to indicate errors. This is no longer needed.

Userland/Libraries/LibSQL/AST/AST.h

@@ -299,7 +299,6 @@ private:
 
 struct ExecutionContext {
     NonnullRefPtr<Database> database;
-    Optional<Result> result;
     class Statement const* statement;
     Tuple* current_row { nullptr };
 };