[BUG] [Zone Decommission] GET response has redundant field

[imRishN]

Describe the bug
The current GET API call to get the decommission state is -
curl -X GET "localhost:9202/_cluster/decommission/awareness/_status?pretty"

{
	"awareness" : {
		"zone" : "zone-1"
	},
	"status" : "SUCCESSFUL"
}

Here, since awareness is already mentioned in GET path, the awareness field in GET response looks redundant.

To Reproduce
Steps to reproduce the behavior:

1. Decommission an attribute - curl -X PUT "localhost:9201/_cluster/decommission/awareness/zone/zone-2?pretty"
2. Get Decommission status - curl -X GET "localhost:9201/_cluster/decommission/awareness/_status?pretty"

Expected behavior
We can simplify the response to just have attribute name and status as field. Something like -

> curl -X GET "localhost:9202/_cluster/decommission/awareness/_status?pretty"
{
  "zone" : "zone-1",
  "status" : "SUCCESSFUL"
}

[imRishN]

Created PR to fix the same - #4751

[imRishN]

Closing issue, PR merged now