Deprecate step param in BlocksByRange RPC request (#3275)

## Issue Addressed

Deprecates the step parameter in the blocks by range request

## Proposed Changes

- Modifies the BlocksByRangeRequest type to remove the step parameter and everywhere we took it into account before
- Adds a new type to still handle coding and decoding of requests that use the parameter

## Additional Info
I went with a deprecation over the type itself so that requests received outside `lighthouse_network` don't even need to deal with this parameter. After the deprecation period just removing the Old blocks by range request should be straightforward

beacon_node/network/src/sync/range_sync/batch.rs

@@ -199,7 +199,6 @@ impl<T: EthSpec, B: BatchConfig> BatchInfo<T, B> {
         BlocksByRangeRequest {
             start_slot: self.start_slot.into(),
             count: self.end_slot.sub(self.start_slot).into(),
-            step: 1,
         }
     }