Allow bump fee on transactions without RBF sequence

[notmandatory]

This should be an easy change, the following lines need to be removed from Wallet::build_fee_bump:
https://github.com/bitcoindevkit/bdk/blob/7969898c732a3a387a932a61b1719cedcb6fae3c/crates/wallet/src/wallet/mod.rs#L1636-L1644

Also related to this discussion (probably warrants a separate issue?) is how the new rules impact the bump fee transaction builder. I'm not sure if the current builder requires a transaction signals for RBF to then be used by the replace by fee txbuilder?

_Originally posted by @thunderbiscuit in #66

[aagbotemi]

@notmandatory kindly assign this task to me, I would like to work on it.

Removing RBF sequence check is that it provides more flexibility even when RBF is not enabled at the point of creating transaction, it allows fee bumping for transactions that weren't originally created with RBF signaling. While Bitcoin Core v28.0 has changed -mempoolfullrbf default value is 1 enabling full RBF by default bitcoin core pr

I would also like to clarify that removing the check means that even if a transaction wasn’t meant to be replaceable, the fee bump builder could still operate on it.