data-source/aws_iam_server_certificate: Fixes for tfproviderlint R002

Reference: #9952

Remove extraneous pointer value dereferences, which cause potential panics and are extraneous as `Set()` automatically handles pointer types including when `nil`.

Previously:

```
aws/data_source_aws_iam_server_certificate.go:142:15: R002: ResourceData.Set() pointer value dereference is extraneous
aws/data_source_aws_iam_server_certificate.go:143:16: R002: ResourceData.Set() pointer value dereference is extraneous
aws/data_source_aws_iam_server_certificate.go:144:16: R002: ResourceData.Set() pointer value dereference is extraneous
```

Also removes extraneous time conversion function.

Output from acceptance testing:

```
--- PASS: TestAccAWSDataSourceIAMServerCertificate_matchNamePrefix (4.36s)
--- PASS: TestAccAWSDataSourceIAMServerCertificate_basic (16.79s)
--- PASS: TestAccAWSDataSourceIAMServerCertificate_path (17.37s)
```

aws/data_source_aws_iam_server_certificate.go

@@ -139,9 +139,9 @@ func dataSourceAwsIAMServerCertificateRead(d *schema.ResourceData, meta interfac
 
 	metadata := metadatas[0]
 	d.SetId(*metadata.ServerCertificateId)
-	d.Set("arn", *metadata.Arn)
-	d.Set("path", *metadata.Path)
-	d.Set("name", *metadata.ServerCertificateName)
+	d.Set("arn", metadata.Arn)
+	d.Set("path", metadata.Path)
+	d.Set("name", metadata.ServerCertificateName)
 	if metadata.Expiration != nil {
 		d.Set("expiration_date", metadata.Expiration.Format(time.RFC3339))
 	}

aws/data_source_aws_iam_server_certificate_test.go

@@ -13,23 +13,19 @@ import (
 	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 )
 
-func timePtr(t time.Time) *time.Time {
-	return &t
-}
-
 func TestResourceSortByExpirationDate(t *testing.T) {
 	certs := []*iam.ServerCertificateMetadata{
 		{
 			ServerCertificateName: aws.String("oldest"),
-			Expiration:            timePtr(time.Now()),
+			Expiration:            aws.Time(time.Now()),
 		},
 		{
 			ServerCertificateName: aws.String("latest"),
-			Expiration:            timePtr(time.Now().Add(3 * time.Hour)),
+			Expiration:            aws.Time(time.Now().Add(3 * time.Hour)),
 		},
 		{
 			ServerCertificateName: aws.String("in between"),
-			Expiration:            timePtr(time.Now().Add(2 * time.Hour)),
+			Expiration:            aws.Time(time.Now().Add(2 * time.Hour)),
 		},
 	}
 	sort.Sort(certificateByExpiration(certs))