Reference: #1145
Reference: hashicorp/terraform#32463

Terraform 1.3.8 and later now correctly handles null values for single nested blocks. This means Terraform will now report differences between a null block and known block with null values. This SDK only supported single nested blocks via its timeouts functionality.

This change is a very targeted removal of any potential `timeouts` block values in a resource state from the `ImportResourceState` RPC. Since configuration is not available during that RPC, it is never valid to return any data beyond null for that block. This will prevent unexpected differences on the first plan after import, where Terraform will report the block removal for configurations which do not contain the block.

New unit test failure prior to code updates:

```
--- FAIL: TestImportResourceState_Timeouts_Removed (0.00s)
    /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/schema/grpc_provider_test.go:1159: unexpected difference:   cty.Value(
        - 	{
        - 		ty: cty.Type{typeImpl: cty.typeObject{AttrTypes: map[string]cty.Type{...}}},
        - 		v:  map[string]any{"id": string("test"), "string_attribute": nil, "timeouts": nil},
        - 	},
        + 	{
        + 		ty: cty.Type{typeImpl: cty.typeObject{AttrTypes: map[string]cty.Type{...}}},
        + 		v: map[string]any{
        + 			"id":               string("test"),
        + 			"string_attribute": nil,
        + 			"timeouts":         map[string]any{"create": nil, "read": nil},
        + 		},
        + 	},
          )
```