Remove support for GET _aliases

[dadoonet]

GET /{index}/_aliases/{name} is supposed to filter aliases by index and/or by name.

But if you want to filter only by alias name, it gives you back all indices:

curl -XDELETE localhost:9200/foo,bar?pretty
curl -XPUT localhost:9200/foo?pretty
curl -XPUT localhost:9200/bar?pretty
curl -XPOST localhost:9200/_aliases?pretty -d '{
    "actions" : [
        { "add" : { "index" : "foo", "alias" : "myalias" } }
    ]
}'

# Correct
curl -XGET localhost:9200/_aliases?pretty
# {
#   "bar" : {
#     "aliases" : { }
#   },
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }

# Correct
curl -XGET localhost:9200/foo/_aliases?pretty
# {
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }

# Incorrect
curl -XGET localhost:9200/_aliases/myalias?pretty
# {
#   "bar" : {
#     "aliases" : { }
#   },
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }


# Correct
curl -XGET localhost:9200/_alias?pretty
# {
#   "bar" : {
#     "aliases" : { }
#   },
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }

# Correct
curl -XGET localhost:9200/foo/_alias?pretty
# {
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }

# Correct
curl -XGET localhost:9200/_alias/myalias?pretty
# {
#   "foo" : {
#     "aliases" : {
#       "myalias" : { }
#     }
#   }
# }

We don't document GET _aliases anymore so may be we should remove its support as we can do the same thing with GET _alias API.

Might be related to #4743
Related to elastic/elasticsearch-js#331

[clintongormley]

There used to be a difference between GET _alias and GET _aliases (the details of which escape me) which Kibana depended on, but I think this has been resolved. @rashidkpc could you confirm?

[rashidkpc]

@spalger you were looking at this the other day. Where did we decide to go with it?

[spalger]

I was looking at this from the perspective of removing it from the client, not sure how it relates to Kibana.

[javanna]

this appears to be a duplicate of #13906. +1 for removing asap, especially given that filtering aliases by name doesn't work.

[dadoonet]

Thank you @javanna. For whatever reason I did not find this issue. Closing this then.