azurerm_dns_ptr_record - refactoring to match the DNS updates

[tombuildsstuff]

Removing the computed etag field - from what I can see we shouldn't be supporting it (this may need to be marked as Removed first instead?)

Tests pass:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -timeout 120m -run TestAccAzureRMDnsPtr
=== RUN   TestAccAzureRMDnsPtrRecord_importBasic
--- PASS: TestAccAzureRMDnsPtrRecord_importBasic (69.87s)
=== RUN   TestAccAzureRMDnsPtrRecord_importWithTags
--- PASS: TestAccAzureRMDnsPtrRecord_importWithTags (89.69s)
=== RUN   TestAccAzureRMDnsPtrRecord_basic
--- PASS: TestAccAzureRMDnsPtrRecord_basic (68.51s)
=== RUN   TestAccAzureRMDnsPtrRecord_updateRecords
--- PASS: TestAccAzureRMDnsPtrRecord_updateRecords (82.06s)
=== RUN   TestAccAzureRMDnsPtrRecord_withTags
--- PASS: TestAccAzureRMDnsPtrRecord_withTags (102.56s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	412.721s

[grubernaut]

LGTM!

[ghost]

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