Remove stale comments

The two fields this comment references are not `Option` type. This
comment seems to be stale.

Author: tcharding

src/wallet/tx_builder.rs

@@ -119,9 +119,6 @@ impl TxBuilderContext for BumpFee {}
 #[derive(Debug)]
 pub struct TxBuilder<'a, B, D, Cs, Ctx> {
     pub(crate) wallet: &'a Wallet<B, D>,
-    // params and coin_selection are Options not becasue they are optionally set (they are always
-    // there) but because `.finish()` uses `Option::take` to get an owned value from a &mut self.
-    // They are only `None` after `.finish()` is called.
     pub(crate) params: TxParams,
     pub(crate) coin_selection: Cs,
     pub(crate) phantom: PhantomData<Ctx>,