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. This is partially 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"]
  }
}

were previously supported and will need to be replaced with

curl -XPUT localhost:9200/index -d '{
  "aliases" : {
    "alias1": {}
  }
}

Closes #5545