fix: remove server unimplemented GrpcStorageImpl#getServiceAccount (#2718)

Partial revert of 96b6330

Author: BenWhitehead

google-cloud-storage/src/main/java/com/google/cloud/storage/GrpcStorageImpl.java

@@ -26,7 +26,6 @@
 import static com.google.cloud.storage.StorageV2ProtoUtils.objectAclEntityOrAltEq;
 import static com.google.cloud.storage.Utils.bucketNameCodec;
 import static com.google.cloud.storage.Utils.ifNonNull;
-import static com.google.cloud.storage.Utils.projectNameCodec;
 import static com.google.common.base.MoreObjects.firstNonNull;
 import static java.util.Objects.requireNonNull;
 
@@ -90,7 +89,6 @@
 import com.google.storage.v2.GetBucketRequest;
 import com.google.storage.v2.GetNotificationConfigRequest;
 import com.google.storage.v2.GetObjectRequest;
-import com.google.storage.v2.GetServiceAccountRequest;
 import com.google.storage.v2.ListBucketsRequest;
 import com.google.storage.v2.ListNotificationConfigsRequest;
 import com.google.storage.v2.ListNotificationConfigsResponse;
@@ -1390,16 +1388,7 @@ public List<Boolean> testIamPermissions(
 
   @Override
   public ServiceAccount getServiceAccount(String projectId) {
-    GetServiceAccountRequest req =
-        GetServiceAccountRequest.newBuilder()
-            .setProject(projectNameCodec.encode(projectId))
-            .build();
-    GrpcCallContext retryContext = Retrying.newCallContext();
-    return Retrying.run(
-        getOptions(),
-        retryAlgorithmManager.getFor(req),
-        () -> storageClient.getServiceAccountCallable().call(req, retryContext),
-        codecs.serviceAccount());
+    return CrossTransportUtils.throwHttpJsonOnly(Storage.class, "getServiceAccount");
   }
 
   @Override

google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java

@@ -4781,7 +4781,7 @@ List<Boolean> testIamPermissions(
    * @return the service account associated with this project
    * @throws StorageException upon failure
    */
-  @TransportCompatibility({Transport.HTTP, Transport.GRPC})
+  @TransportCompatibility({Transport.HTTP})
   ServiceAccount getServiceAccount(String projectId);
 
   /**

google-cloud-storage/src/test/java/com/google/cloud/storage/conformance/retry/RpcMethodMappings.java

@@ -2114,6 +2114,7 @@ static final class ServiceAccount {
       private static void get(ArrayList<RpcMethodMapping> a) {
         a.add(
             RpcMethodMapping.newBuilder(59, serviceaccount.get)
+                .withApplicable(TestRetryConformance.transportIs(Transport.HTTP))
                 .withTest(
                     (ctx, c) ->
                         ctx.map(

google-cloud-storage/src/test/java/com/google/cloud/storage/it/ITServiceAccountTest.java

@@ -32,7 +32,7 @@
 @RunWith(StorageITRunner.class)
 @CrossRun(
     backends = {Backend.PROD},
-    transports = {Transport.HTTP, Transport.GRPC})
+    transports = {Transport.HTTP})
 public class ITServiceAccountTest {
 
   private static final String SERVICE_ACCOUNT_EMAIL_SUFFIX =