fixUnusedIdentifiers removes unused function parameter in ES6 syntax

**Description**

Version: ^12.0.0

When I call fixUnusedIdentifiers(), part of the function argument is removed which in ES6 syntax.

**To Reproduce**

```ts
// before
testUnUsedObject={
beforeFetch1(a, b) {
return b
},
beforeFetch2: (a, b) => {
return b
},
}
// after call fixUnusedIdentifiers()
testUnUsedObject={
beforeFetch1(b) {
return b
},
beforeFetch2: (a, b) => {
return b
},
}
```

**Expected behavior**
beforeFetch1 and beforeFetch2 behave the same

Start by locating fixUnusedIdentifiers and reproduce the issue with the beforeFetch1 method and beforeFetch2 arrow-function examples from the report. Compare how unused parameters are handled for each ES6 form; done means both forms preserve equivalent behavior after the fix, with the provided reproduction covered by a regression test.

Written by the indexing model from the issue text.