psbt: Delete TODO comments

Development for `psbt` has move to another repo, these TODO comments are
over there alread, lets just remove them from `rust-bitcoin` as part of
an effort to remove TODOs from the codebase.

bitcoin/src/psbt/map/input.rs

@@ -97,7 +97,6 @@ pub struct Input {
     /// The finalized, fully-constructed scriptWitness with signatures and any
     /// other scripts necessary for this input to pass validation.
     pub final_script_witness: Option<Witness>,
-    /// TODO: Proof of reserves commitment
     /// RIPEMD160 hash to preimage map.
     #[cfg_attr(feature = "serde", serde(with = "crate::serde_utils::btreemap_byte_values"))]
     pub ripemd160_preimages: BTreeMap<ripemd160::Hash, Vec<u8>>,

bitcoin/src/psbt/serialize.rs

@@ -321,7 +321,6 @@ impl Serialize for (Vec<TapLeafHash>, KeySource) {
     fn serialize(&self) -> Vec<u8> {
         let mut buf = Vec::with_capacity(32 * self.0.len() + key_source_len(&self.1));
         self.0.consensus_encode(&mut buf).expect("Vecs don't error allocation");
-        // TODO: Add support for writing into a writer for key-source
         buf.extend(self.1.serialize());
         buf
     }