Auto merge of #7196 - frewsxcv:double-unsafe, r=jdm

Avoid marking codegen method bodies as unsafe twice

`CGAbstractMethod` takes a couple boolean parameters, among others:

* `extern`: will mark the method as `unsafe` and `extern`
* `unsafe`: will wrap the method body in an `unsafe` block

Passing both as `True` should not mark it as `unsafe` twice.

Example from a generated `HTMLCollectionBinding.rs`:

Before:

```
unsafe extern fn get_length(..) -> u8 {
    unsafe {
        // code here
    }
}
```

After

```
unsafe extern fn get_length(..) -> u8 {
    // code here
}
```

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