Removed support for aliases as part of index settings

Now that we have explicit support for aliases when creating indices and as part of index templates, we may remove support for aliases (only names) as part of index settings. The change is marked as breaking as the following calls:

```
curl -XPUT localhost:9200/index -d '{
  "settings" : {
    "aliases" : [ "alias1"]
  }
}
```

and

```
curl -XPUT localhost:9200/index -d '{
  "settings" : {
    "index.aliases" : [ "alias1"]
  }
}
```

won't be supported anymore and will need to be replaced with

```
curl -XPUT localhost:9200/index -d '{
  "aliases" : {
    "alias1": {}
  }
}
```