I haven't used DNSSEC myself yet but I'm assuming the usual restrictions on KMS key deletion apply. You can't delete KMS keys immediately, only schedule them for deletion with a min 7 days, default 30 days waiting period.
This is to protect you - deleting a customer master key is destructive and potentially dangerous. It deletes the key material and all metadata associated with the CMK, and is irreversible. After a CMK is deleted you can no longer decrypt the data that was encrypted under that CMK, which means that data becomes unrecoverable.
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