Remove legacy max-skip-slots checks (#4403)

## Proposed Changes

Remove `max-skip-slots` checks when processing blocks.
This was legacy code which was previously used in the Medalla testnet to sync to the correct fork.
With the addition of checkpoint sync which allows us to sync to any arbitrary fork, this is no longer a necessary feature, so it has been removed for simplicity.

## Additional Notes
The CLI flag and checks for attestation processing have been retained as it still may have uses in DoS protection.

beacon_node/beacon_chain/src/chain_config.rs

@@ -17,8 +17,7 @@ pub const FORK_CHOICE_LOOKAHEAD_FACTOR: u32 = 24;
 
 #[derive(Debug, PartialEq, Eq, Clone, Deserialize, Serialize)]
 pub struct ChainConfig {
-    /// Maximum number of slots to skip when importing a consensus message (e.g., block,
-    /// attestation, etc).
+    /// Maximum number of slots to skip when importing an attestation.
     ///
     /// If `None`, there is no limit.
     pub import_max_skip_slots: Option<u64>,