Removed all disable_forks (#2925)

#2923 

Which issue # does this PR address?
There's a redundant field on the BeaconChain called disabled_forks that was once part of our fork-aware networking (#953) but which is no longer used and could be deleted. so Removed all references to disabled_forks so that the code compiles and git grep disabled_forks returns no results.

## Proposed Changes

Please list or describe the changes introduced by this PR.
Removed all references of disabled_forks


Co-authored-by: Divma <26765164+divagant-martian@users.noreply.github.com>

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -336,8 +336,6 @@ pub struct BeaconChain<T: BeaconChainTypes> {
     pub early_attester_cache: EarlyAttesterCache<T::EthSpec>,
     /// A cache used to keep track of various block timings.
     pub block_times_cache: Arc<RwLock<BlockTimesCache>>,
-    /// A list of any hard-coded forks that have been disabled.
-    pub disabled_forks: Vec<String>,
     /// Sender given to tasks, so that if they encounter a state in which execution cannot
     /// continue they can request that everything shuts down.
     pub shutdown_sender: Sender<ShutdownReason>,

beacon_node/beacon_chain/src/builder.rs

@@ -84,7 +84,6 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     validator_pubkey_cache: Option<ValidatorPubkeyCache<T>>,
     spec: ChainSpec,
     chain_config: ChainConfig,
-    disabled_forks: Vec<String>,
     log: Option<Logger>,
     graffiti: Graffiti,
     slasher: Option<Arc<Slasher<T::EthSpec>>>,
@@ -122,7 +121,6 @@ where
             slot_clock: None,
             shutdown_sender: None,
             head_tracker: None,
-            disabled_forks: Vec::new(),
             validator_pubkey_cache: None,
             spec: TEthSpec::default_spec(),
             chain_config: ChainConfig::default(),
@@ -184,13 +182,6 @@ where
         self.log = Some(log);
         self
     }
-
-    /// Sets a list of hard-coded forks that will not be activated.
-    pub fn disabled_forks(mut self, disabled_forks: Vec<String>) -> Self {
-        self.disabled_forks = disabled_forks;
-        self
-    }
-
     /// Attempt to load an existing eth1 cache from the builder's `Store`.
     pub fn get_persisted_eth1_backend(&self) -> Result<Option<SszEth1>, String> {
         let store = self
@@ -764,7 +755,6 @@ where
             validator_pubkey_cache: TimeoutRwLock::new(validator_pubkey_cache),
             attester_cache: <_>::default(),
             early_attester_cache: <_>::default(),
-            disabled_forks: self.disabled_forks,
             shutdown_sender: self
                 .shutdown_sender
                 .ok_or("Cannot build without a shutdown sender.")?,