Remove aws_vpc_dhcp_options if not found

[ewbankkit]

Fixes the second part of #13489.
If DHCP Options can't be found during the Read pass, remove.

Acceptance test:

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDHCPOptions_deleteOptions'

[stack72]

LGTM! Thanks for this :)

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDHCPOptions_'                                              ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/17 01:29:17 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDHCPOptions_ -timeout 120m
=== RUN   TestAccAWSDHCPOptions_importBasic
--- PASS: TestAccAWSDHCPOptions_importBasic (25.63s)
=== RUN   TestAccAWSDHCPOptions_basic
--- PASS: TestAccAWSDHCPOptions_basic (25.22s)
=== RUN   TestAccAWSDHCPOptions_deleteOptions
--- PASS: TestAccAWSDHCPOptions_deleteOptions (19.27s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	70.179s

[ghost]

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