Remove SchemaConfigModeAttr

[modular-magician]

fixes hashicorp/terraform-provider-google#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composer: `ip_allocation_policy = []` in `google_composer_environment` is no longer valid configuration.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compute: `secondary_ip_ranges = []` in `google_compute_subnetwork` is no longer valid configuration.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compute: `guest_accelerator = []` is no longer valid configuration in `google_compute_instance`.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container: `guest_accelerator = []` is no longer valid configuration in `google_container_cluster` and `google_container_node_pool`.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container: `guest_accelerator.gpu_driver_installation_config = []` and `guest_accelerator.gpu_sharing_config = []` are no longer valid configuration in `google_container_cluster` and `google_container_node_pool`.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

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/