Remove key_size() method from Column trait

This helper simply called std::mem::size_of<Self::Index>(). However, all
of the underlying functions that create keys manually copy fields into a
byte array. The fields are copied in end-to-end whereas size_of() might
include alignment bytes.

That is, a (u64, u32) only has 12 bytes of "data", but it would have
size 16 due to the 4 alignment padding bytes that would be added to get
the u32 (size 4) aligned with the u64 (size 8).

ledger/src/blockstore_db.rs

@@ -719,10 +719,6 @@ impl Rocks {
 pub trait Column {
     type Index;
 
-    fn key_size() -> usize {
-        std::mem::size_of::<Self::Index>()
-    }
-
     fn key(index: Self::Index) -> Vec<u8>;
     fn index(key: &[u8]) -> Self::Index;
     // This trait method is primarily used by `Database::delete_range_cf()`, and is therefore only