inferred unused function names are not removed

**Describe the bug**

Follow up of #842 -

Function names created by function name transformer of es2015 aren't removed when unused.

**To Reproduce**

Minimal code to reproduce the bug

```js
function foo() {
return {
a() {}
}
}
```

**Actual Output**

If there is no Error thrown,

```js
function foo() {
return {
a: function b() {}
};
}
```

**Expected Output**

```js
function foo() {
return {
a: function() {}
};
}
```

**Configuration**

`babel-minify-cli`

babel-minify version: master after #841

babel version : `7.0.0-beta.46`

babel-minify-config:

```json5
{
removeConsole: true,
keepFnNames: true
}
```

babelrc:

```json5
{
plugins: [],
presets: []
}
```

**Possible solution**

Apply the same fix as in #842 - force register the binding (because this binding has been skipped by babel)