Remove is_swapping from registration details

[HCastano]

This was originally used for swapping threshold keys, but it doesn't look like it's
used anymore.

From conversations with peeps it does seem like this is something that could come back in
the future (potentially with Centralized Key Generation), but it would have to be
redesigned anyways.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
entropy-core	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 19, 2023 10:28pm

[HCastano]

@JesseAbram the user::tests::test_register_with_private_key_visibility test is flaky for me, so I'm having a hard time telling if it's failing due to this PR.

If I run it back to back on master it passes the first time and then fails. I have the same node running between both test runs (cargo run -p entropy --dev).

Edit: It has to do with a requirement to run the node in release mode. I opened another PR to fix this