Allow bump fee on transactions without RBF sequence

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 https://github.com/bitcoindevkit/bdk_wallet/issues/66