tag field in timer appears to be redundant

In the **timer** component, there is no assigned value to the **tag** field anywhere, so can we delete the **tag** field and its judgment logic? Because it is always **zero**.

```go
// timer.go

// Model of the timer component.
type Model struct {
...
tag int
...
}
```

Start in timer.go and inspect the timer Model's tag field and its judgment logic, then search for remaining tag references. Done means the redundant tag field and all of its judgment logic are removed without leaving references behind.

Written by the indexing model from the issue text.