Remove Hrp::is_empty

This function always returns `false`, it is pointless - remove it.

src/primitives/hrp.rs

@@ -178,12 +178,9 @@ impl Hrp {
     ///
     /// Guaranteed to be between 1 and 83 inclusive.
     #[inline]
+    #[allow(clippy::len_without_is_empty)] // HRP is never empty.
     pub fn len(&self) -> usize { self.size }
 
-    /// Always false, the human-readable part is guaranteed to be between 1-83 characters.
-    #[inline]
-    pub fn is_empty(&self) -> bool { false }
-
     /// Returns `true` if this [`Hrp`] is valid according to the bips.
     ///
     /// [BIP-173] states that the HRP must be either "bc" or "tb".