Remove code comment on is_x_only_key

The `is_x_only_key` trait method is used for more than one thing, the
code comment is either stale, not exhaustive, or wrong. Let's just
remove it.

src/lib.rs

@@ -159,8 +159,6 @@ pub trait MiniscriptKey: Clone + Eq + Ord + fmt::Debug + fmt::Display + hash::Ha
     fn is_uncompressed(&self) -> bool { false }
 
     /// Returns true if the key is an x-only pubkey, defaults to `false`.
-    // This is required to know what in DescriptorPublicKey to know whether the inner
-    // key in allowed in descriptor context
     fn is_x_only_key(&self) -> bool { false }
 
     /// Returns the number of different derivation paths in this key, defaults to `0`.