Fix MySQL deletion subnet error

[vimtor]

if you try to remove a mysql database via sst remove you get:

MyDatabase sst:aws:Mysql → MyDatabaseSubnetGroup aws:rds:SubnetGroup
sdk-v2/provider2.go:572: sdk.helper_schema: deleting RDS Subnet Group (aws-mysql-production-mydatabasesubnetgroup-zntkucex): operation error RDS: DeleteDBSubnetGroup, https response error StatusCode: 400, RequestID: b65c37b8-63ff-4f90-939e-5a52f309910f, InvalidDBSubnetGroupStateFault: Cannot delete the subnet group 'aws-mysql-production-mydatabasesubnetgroup-zntkucex' because at least one database instance: aws-mysql-production-mydatabaseinstance-bbubmsmv is still using it.: provider=aws@7.20.0