Remove SchemaConfigModeAttr

[modular-magician]

fixes #12824

Compute subnetwork preparation change: GoogleCloudPlatform/magic-modules#11410
Guest Accelerator preparation change: GoogleCloudPlatform/magic-modules#11425

Composer's ip_allocation_policy did not actually send an empty list when specifying an empty block, and it is create-only. Therefore removing configModeAttr doesn't make a difference

google_compute_instance_from_template and google_compute_instance_from_machine_image functionality for empty blocks is being removed to prevent accidental breaking changes for new subfields from the parent google_compute_instance

Release Note Template for Downstream PRs (will be copied)

composer: `ip_allocation_policy = []` in `google_composer_environment` is no longer valid configuration. Removing the field from configuration should not produce a diff.

compute: `secondary_ip_ranges = []` in `google_compute_subnetwork` is no longer valid configuration. To set an explicitly empty list, use `send_secondary_ip_range_if_empty` and completely remove `secondary_ip_range` from config.

compute: `guest_accelerator = []` is no longer valid configuration in `google_compute_instance`. To explicitly set an empty list of objects, set guest_accelerator.count = 0.

compute: `google_compute_instance_from_template` and `google_compute_instance_from_machine_image` `network_interface.alias_ip_range, network_interface.access_config, attached_disk, guest_accelerator, service_account, scratch_disk` can no longer be set to an empty block `[]`. Removing the fields from configuration should not produce a diff.

container: `guest_accelerator = []` is no longer valid configuration in `google_container_cluster` and `google_container_node_pool`. To explicitly set an empty list of objects, set guest_accelerator.count = 0.

container: `guest_accelerator.gpu_driver_installation_config = []` and `guest_accelerator.gpu_sharing_config = []` are no longer valid configuration in `google_container_cluster` and `google_container_node_pool`. Removing the fields from configuration should not produce a diff.

Derived from GoogleCloudPlatform/magic-modules#11506

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