Remove trait ComputeSighash

[ValuedMammal]

This PR does some cleanup of the bdk_wallet signer module most notably by removing the internal trait ComputeSighash and replacing old code for computing the sighash (for legacy and segwit context) with a single method Psbt::sighash_ecdsa. The logic for computing the taproot sighash is unchanged and extracted to a new helper function compute_tap_sighash.

• [x] Unimplement ComputeSighash
• [x] Try de-duplicating code by using Psbt::sighash_ecdsa. see https://github.com/bitcoindevkit/bdk/pull/1023#discussion_r1263140218

fixes #1038

Notes to the reviewers

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• [x] I've signed all my commits
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• [x] I ran cargo fmt and cargo clippy before committing