Merge rust-bitcoin/rust-bitcoin#1063: Remove duplicate must_use

bea5569 Remove duplicate must_use (Tobin C. Harding)

Pull request description:

  Clippy emits:

   warning: this function has an empty `#[must_use]` attribute, but returns a type already marked as `#[must_use]`

  This is because the return type of the function `legacy_encode_signing_data_to` is `EncodeSigningDataResult` which is
  already marked as `must_use`. There is no need to have `must_use` on the function also.

  ##

  Hey apoelstra, this PR is needed before any other PR can get a green CI run. The PR that introduced this warning must have had a green CI run before the 'add clippy to CI' PR merged allowing it to merge without re-running CI.

ACKs for top commit:
  apoelstra:
    ACK bea5569
  Kixunil:
    ACK bea5569

Tree-SHA512: 27b606685bc2d5317a5e332431511af9b1017bf58462c283b322e5b6faf645fd8e006455af3d6888ab2c3c680b11e4b89f538f26440f4f1422e0daabc61825ac

Author: apoelstra

src/util/sighash.rs

@@ -647,7 +647,6 @@ impl<R: Deref<Target = Transaction>> SighashCache<R> {
 
     /// Encodes the legacy signing data for any flag type into a given object implementing a
     /// [`std::io::Write`] trait. Internally calls [`Transaction::encode_signing_data_to`].
-    #[must_use]
     pub fn legacy_encode_signing_data_to<Write: io::Write, U: Into<u32>>(
         &self,
         mut writer: Write,