Add expense/income deletion

src/main/java/com/faf223/expensetrackerfaf/service/ExpenseService.java

@@ -40,4 +40,8 @@ public class ExpenseService implements ITransactionService {
     public Expense getTransactionById(long id) {
         return expenseRepository.findById(id).orElse(null);
     }
+
+    public void deleteExpenseById(long id) {
+        expenseRepository.deleteById(id);
+    }
 }