Remove GET `_aliases` api in favour for GET `_alias` api

Currently there are two get aliases apis that both have the same functionality, but have a different response structure. The reason for having 2 apis is historic.

The GET _alias api was added in 0.90.x and is more efficient since it only sends the needed alias data from the cluster state between the master node and the node that received the request. In the GET _aliases api the complete cluster state is send to the node that received the request and then the right information is filtered out and send back to the client.

The GET _aliases api should be removed in favour for the alias api

Closes to #4539