Delete orphaned owned TXT record

[ruudk]

Fixes #1502

When using the TXT registry it can sometimes happen that the A record is removed but the TXT record not. This could be caused by a human for example.

In that case, external-dns will try to recreate the missing A record. It will blindly also create a new TXT record. But that record already exists. The operation will fail and the A record won't be created until the TXT record is also removed. See #1502 for full details.

With this change, the existing TXT record will be removed before creating it again.

I will add tests once it is agreed that this is the right way to go.

Todo

• Add tests

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@ruudk: PR needs rebase.

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