Drop overflow cache (#5891)

* Drop overflow cache

* Update docs

* Update beacon_node/store/src/lib.rs

* Update data_availability_checker.rs

* Lint

beacon_node/store/src/lib.rs

@@ -289,7 +289,7 @@ impl DBColumn {
     /// This function returns the number of bytes used by keys in a given column.
     pub fn key_size(self) -> usize {
         match self {
-            Self::OverflowLRUCache => 33, // See `OverflowKey` encode impl.
+            Self::OverflowLRUCache => 33, // DEPRECATED
             Self::BeaconMeta
             | Self::BeaconBlock
             | Self::BeaconState