Remove unused LPSPI I/O helpers

Similar to the parent commit, these are prototyping leftovers from
f4a369e that beta clippy is picking up. However, I'm committing this
separately to make it easy to revert. I figured these could be helpful
for realizing EH1 traits, but I never tested them.

src/common/lpspi.rs

@@ -1432,15 +1432,6 @@ where
     }
 }
 
-/// Transmits dummy values.
-struct TransmitDummies;
-
-impl TransmitData for TransmitDummies {
-    fn next_word(&mut self, _: BitOrder) -> u32 {
-        u32::MAX
-    }
-}
-
 /// Receive data into a buffer.
 struct ReceiveBuffer<'a, W> {
     /// The write position.
@@ -1495,13 +1486,6 @@ where
     }
 }
 
-/// Receive dummy data.
-struct ReceiveDummies;
-
-impl ReceiveData for ReceiveDummies {
-    fn next_word(&mut self, _: u32) {}
-}
-
 /// Computes how may Ws fit inside a LPSPI word.
 const fn per_word<W: Word>() -> usize {
     core::mem::size_of::<u32>() / core::mem::size_of::<W>()