provider/aws: aws_vpn_gateway should be removed from state when in deleted state

[stack72]

Fixes #7859

When a VPN Gateway has been manually deleted, we should expect it to be
added back to the plan

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSVpnGateway_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSVpnGateway_
-timeout 120m
=== RUN   TestAccAWSVpnGateway_importBasic
--- PASS: TestAccAWSVpnGateway_importBasic (247.94s)
=== RUN   TestAccAWSVpnGateway_basic
--- PASS: TestAccAWSVpnGateway_basic (409.50s)
=== RUN   TestAccAWSVpnGateway_reattach
--- PASS: TestAccAWSVpnGateway_reattach (211.33s)
=== RUN   TestAccAWSVpnGateway_delete
--- PASS: TestAccAWSVpnGateway_delete (121.10s)
=== RUN   TestAccAWSVpnGateway_tags
--- PASS: TestAccAWSVpnGateway_tags (125.38s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws
1115.274s

[stack72]

Ideally, this should be in 0.7! Will follow up with an acceptance test to show that this works as expected

[phinze]

@stack72 this looks pretty low risk - LGTM for merge in 0.7

[ghost]

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