chore: cleanup extraneous executor handling

Currently, we control the full lifecycle of the instance of StorageSettings and can ensure it will never need a backup executor service.

Author: BenWhitehead

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

@@ -27,7 +27,6 @@
 import com.google.api.core.ObsoleteApi;
 import com.google.api.gax.core.CredentialsProvider;
 import com.google.api.gax.core.FixedCredentialsProvider;
-import com.google.api.gax.core.InstantiatingExecutorProvider;
 import com.google.api.gax.core.NoCredentialsProvider;
 import com.google.api.gax.grpc.GrpcCallSettings;
 import com.google.api.gax.grpc.GrpcInterceptorProvider;
@@ -821,14 +820,7 @@ public Storage create(StorageOptions options) {
           Opts<UserProject> defaultOpts = t.y();
 
           ScheduledExecutorService executor =
-              Utils.firstNonNull(
-                      storageSettings::getBackgroundExecutorProvider,
-                      () -> {
-                        // TODO: if we make it to here, ensure we track the need to shutdown the
-                        //   executor separate from StorageClient
-                        return InstantiatingExecutorProvider.newBuilder().build();
-                      })
-                  .getExecutor();
+              storageSettings.getBackgroundExecutorProvider().getExecutor();
           RetryContextProvider retryContextProvider =
               RetryContext.providerFrom(
                   executor,