Fix issue with AWS secrets manager override default stage

Before change
```
⇶  packer build amazon-ebs_secretsmanager_shell-local.json
Error:
template: root:1:3: executing "root" at <aws_secretsmanager `packer/test/keys`

`shell`>: error calling aws_secretsmanager: ResourceNotFoundException: Secrets
Manager can't find the specified secret value for staging label: shell

2020/10/30 12:53:40 [INFO] (telemetry) Finalizing.
template: root:1:3: executing "root" at <aws_secretsmanager `packer/test/keys`
`shell`>: error calling aws_secretsmanager: ResourceNotFoundException: Secrets
Manager can't find the specified secret value for staging label: shell

⇶  packer build amazon-ebs_secretsmanager_shell-local.json.pkr.hcl
Error: Error in function call

  on amazon-ebs_secretsmanager_shell-local.json.pkr.hcl line 28:
  (source code not available)

Call to function "aws_secretsmanager" failed: ResourceNotFoundException: Secrets
Manager can't find the specified secret value for staging label: home.

```

After change
```
⇶  packer.test build amazon-ebs_secretsmanager_shell-local.json
null: output will be in this color.

==> null: Running local shell script: /tmp/packer-shell463393820
    null: boo
    null: keys:powershell
Build 'null' finished after 8 milliseconds 225 microseconds.

==> Wait completed after 8 milliseconds 319 microseconds

==> Builds finished. The artifacts of successful builds are:
--> null: Did not export anything. This is the null builder

⇶  packer.test build amazon-ebs_secretsmanager_shell-local.json.pkr.hcl
null.autogenerated_1: output will be in this color.

==> null.autogenerated_1: Running local shell script: /tmp/packer-shell834410761
    null.autogenerated_1: boo
    null.autogenerated_1: keys:powershell
Build 'null.autogenerated_1' finished after 18 milliseconds 834 microseconds.

==> Wait completed after 18 milliseconds 954 microseconds

==> Builds finished. The artifacts of successful builds are:
--> null.autogenerated_1: Did not export anything. This is the null builder

```

template/interpolate/aws/secretsmanager/secretsmanager.go

@@ -53,9 +53,6 @@ func (c *Client) GetSecret(spec *SecretSpec) (string, error) {
 		SecretId:     aws.String(spec.Name),
 		VersionStage: aws.String("AWSCURRENT"),
 	}
-	if spec.Name != "" {
-		params.VersionStage = aws.String(spec.Key)
-	}
 
 	resp, err := c.api.GetSecretValue(params)
 	if err != nil {