Fix permadiffs in regional health_checks and backend_service

[modular-magician]

Applying #4396 to regional equivalent

log_config is computed on google_compute_backend_service so doing the same on the regional one.

https://github.com/hashicorp/terraform-provider-google/blob/master/google/resource_compute_backend_service.go#L639

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed a perma-diff on `google_compute_region_health_check` when `log_config.enable` is set to false

Derived from GoogleCloudPlatform/magic-modules#4998

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.