internal/provider(test): remove DMS from unusual endpoint tests (#38694)

DMS has been migrated to AWS SDK for Go V2, and thus the corresponding `DMSConn` method to create a V1 client no longer exists. The generic `TestAccProvider_endpoints` test effectively ignores services which use V2 AWS SDK clients with the following conditional in the test check function (`serviceConn` will always return a method name ending in `Conn`):

```go
			methodName := serviceConn(serviceKey)
			method := reflect.ValueOf(providerClient).MethodByName(methodName)
			if !method.IsValid() {
				continue
			}
```

Because this generic test check ignores V2 clients, the DMS case for the unusual endpoints tests has been removed entirely rather than including new conditional logic which would effectively just skip the case before executing any real checks.

```console
% TF_ACC=1 go test -count=1 ./internal/provider/... -run="TestAccProvider_unusualEndpoints"
?       github.com/hashicorp/terraform-provider-aws/internal/provider/fwprovider        [no test files]
ok      github.com/hashicorp/terraform-provider-aws/internal/provider   23.263s
```

internal/provider/provider_acc_test.go

@@ -217,8 +217,7 @@ func TestAccProvider_unusualEndpoints(t *testing.T) {
 	ctx := acctest.Context(t)
 	var provider *schema.Provider
 	unusual1 := unusualEndpoint{"es", "elasticsearch", "http://notarealendpoint"}
-	unusual2 := unusualEndpoint{"databasemigration", "dms", "http://alsonotarealendpoint"}
-	unusual3 := unusualEndpoint{"lexmodelbuildingservice", "lexmodels", "http://kingofspain"}
+	unusual2 := unusualEndpoint{"lexmodelbuildingservice", "lexmodels", "http://kingofspain"}
 
 	resource.ParallelTest(t, resource.TestCase{
 		PreCheck:                 func() { acctest.PreCheck(ctx, t) },
@@ -227,11 +226,10 @@ func TestAccProvider_unusualEndpoints(t *testing.T) {
 		CheckDestroy:             nil,
 		Steps: []resource.TestStep{
 			{
-				Config: testAccProviderConfig_unusualEndpoints(unusual1, unusual2, unusual3),
+				Config: testAccProviderConfig_unusualEndpoints(unusual1, unusual2),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckUnusualEndpoints(ctx, &provider, unusual1),
 					testAccCheckUnusualEndpoints(ctx, &provider, unusual2),
-					testAccCheckUnusualEndpoints(ctx, &provider, unusual3),
 				),
 			},
 		},
@@ -1056,7 +1054,7 @@ resource "aws_s3_bucket" "test" {
 `, endpoint, rName))
 }
 
-func testAccProviderConfig_unusualEndpoints(unusual1, unusual2, unusual3 unusualEndpoint) string {
+func testAccProviderConfig_unusualEndpoints(unusual1, unusual2 unusualEndpoint) string {
 	//lintignore:AT004
 	return acctest.ConfigCompose(testAccProviderConfig_base, fmt.Sprintf(`
 provider "aws" {
@@ -1067,10 +1065,9 @@ provider "aws" {
   endpoints {
     %[1]s = %[2]q
     %[3]s = %[4]q
-    %[5]s = %[6]q
   }
 }
-`, unusual1.fieldName, unusual1.url, unusual2.fieldName, unusual2.url, unusual3.fieldName, unusual3.url))
+`, unusual1.fieldName, unusual1.url, unusual2.fieldName, unusual2.url))
 }
 
 func testAccProviderConfig_useFipsEndpointFlag(rName string) string {