tests/resource/aws_instance: Simplify TestAccAWSInstance_volumeTagsComputed
tests/resource/aws_instance: Simplify TestAccAWSInstance_volumeTagsComputed

The `associate_public_ip_address` argument can be flakey: Frequent test status changes: 21 changes out of 69 invocations. Its not necessary for this test configuration, so removing it. Also cleans up other extraneous test configurations like `count` and explictit `depends_on`.

Previous output from acceptance testing:

```
--- FAIL: TestAccAWSInstance_volumeTagsComputed (128.66s)
    testing.go:538: Step 0 error: After applying this step, the plan was not empty:
...
        DESTROY/CREATE: aws_instance.foo
...
          associate_public_ip_address:               "false" => "true" (forces new resource)
```

Output from acceptance testing:

```
--- PASS: TestAccAWSInstance_volumeTagsComputed (145.77s)
```

Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws