Drop `claimable` from `Balance::claimable_amount_satoshis` fields

Drop `claimable` from `Balance::claimable_amount_satoshis` fields

In Java/TypeScript, we map enums as a base class and each variant
as a class which extends the base. In Java/TypeScript, functions
and fields share the same namespace, which means we cannot have
functions on an enum which have the same name as any fields in any
enum variants.

`Balance`'s `claimable_amount_satoshis` method aliases with fields
in each variant, and thus ultimately doesn't compile in TypeScript.

Because `Balance::claimable_amount_satoshis` has the same name as
the fields, it's also a bit confusing, as it doesn't return the
field for each variant, but sometimes returns zero if we're not
sure we can claim the balance.

Instead, we rename the fields in each enum variant to simply
`amount_satoshis`, to avoid implying that we can definitely claim
the balance.