Remove user secret key from encrypted session token (#652)

User secret key is not really need it to be stored inside the encrypted
session key, since the `change-password` endpoint requires the user to
provide the current `secret key` that password will be used to
initialize a new minio client then we will leverage on the
`SetUser` operation, this api only works with actual user credentials
and not sts credentials

models/principal.go

@@ -44,9 +44,6 @@ type Principal struct {
 	// account access key
 	AccountAccessKey string `json:"accountAccessKey,omitempty"`
 
-	// account secret key
-	AccountSecretKey string `json:"accountSecretKey,omitempty"`
-
 	// actions
 	Actions []string `json:"actions"`
 }