Shift networking configuration (#4426)

## Issue Addressed
Addresses [#4401](https://github.com/sigp/lighthouse/issues/4401)

## Proposed Changes
Shift some constants into ```ChainSpec``` and remove the constant values from code space.

## Additional Info

I mostly used ```MainnetEthSpec::default_spec()``` for getting ```ChainSpec```. I wonder Did I make a mistake about that.


Co-authored-by: armaganyildirak <armaganyildirak@gmail.com>
Co-authored-by: Paul Hauner <paul@paulhauner.com>
Co-authored-by: Age Manning <Age@AgeManning.com>
Co-authored-by: Diva M <divma@protonmail.com>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -217,11 +217,6 @@ pub enum OverrideForkchoiceUpdate {
     AlreadyApplied,
 }
 
-/// The accepted clock drift for nodes gossiping blocks and attestations. See:
-///
-/// https://github.com/ethereum/eth2.0-specs/blob/v0.12.1/specs/phase0/p2p-interface.md#configuration
-pub const MAXIMUM_GOSSIP_CLOCK_DISPARITY: Duration = Duration::from_millis(500);
-
 #[derive(Debug, PartialEq)]
 pub enum AttestationProcessingOutcome {
     Processed,