pdl: Remove '"' prefix and suffix from string

Strings are quotted in '"' in pdl, currently
we keep thoses in the ast.

So

```
custom_field Address : 48 "hci/"
```

ends up in the ast as

```
{
      "kind": "custom_field_declaration",
      "id": "Address",
      "width": 48,
      "function": "\"hci/\""
}
```

To simplify the generator code, remove the quotes to
instead emit

```
{
      "kind": "custom_field_declaration",
      "id": "Address",
      "width": 48,
      "function": "hci/"
}
```

Test: manual
Change-Id: I06fefc06b7f4f740f12dfd68567460015e7b5862