chore(nns): Clean up the migration code to backfill InstallCode hashes (

#3809)

Cleaning up the one-off migration code (the migration code is run after
[this
proposal](https://dashboard.internetcomputer.org/proposal/135063)). For
new proposals, the hashes are computed when they are created.

rs/nns/governance/src/governance.rs

@@ -77,7 +77,6 @@ use ic_base_types::{CanisterId, PrincipalId};
 use ic_cdk::println;
 #[cfg(target_arch = "wasm32")]
 use ic_cdk::spawn;
-use ic_crypto_sha2::Sha256;
 use ic_nervous_system_common::{
     cmc::CMC, ledger, ledger::IcpLedger, NervousSystemError, ONE_DAY_SECONDS, ONE_MONTH_SECONDS,
     ONE_YEAR_SECONDS,
@@ -1865,7 +1864,7 @@ impl Governance {
             env.seed_rng(rng_seed);
         }
 
-        let mut governance = Self {
+        Self {
             heap_data: heap_governance_proto,
             neuron_store: NeuronStore::new_restored((heap_neurons, topic_followee_map)),
             env,
@@ -1877,10 +1876,7 @@ impl Governance {
             neuron_data_validator: NeuronDataValidator::new(),
             minting_node_provider_rewards: false,
             neuron_rate_limits: NeuronRateLimits::default(),
-        };
-        // TODO: Remove after the backfill has been run once.
-        governance.backfill_install_code_hashes();
-        governance
+        }
     }
 
     /// After calling this method, the proto and neuron_store (the heap neurons at least)
@@ -6010,38 +6006,6 @@ impl Governance {
         )
     }
 
-    pub fn backfill_install_code_hashes(&mut self) {
-        for proposal in self.heap_data.proposals.values_mut() {
-            let Some(proposal) = proposal.proposal.as_mut() else {
-                continue;
-            };
-            let Some(Action::InstallCode(install_code)) = proposal.action.as_mut() else {
-                continue;
-            };
-            if install_code.wasm_module_hash.is_none() {
-                if let Some(wasm_module) = install_code.wasm_module.as_ref() {
-                    install_code.wasm_module_hash = Some(Sha256::hash(wasm_module).to_vec());
-                }
-            }
-            if install_code.arg_hash.is_none() {
-                let arg_hash = match install_code.arg.as_ref() {
-                    Some(arg) => {
-                        // We could calculate the hash of an empty arg, but it would be confusing for the
-                        // proposal reviewers, since the arg_hash is the only thing they can see, and it would
-                        // not be obvious that the arg is empty.
-                        if arg.is_empty() {
-                            Some(vec![])
-                        } else {
-                            Some(Sha256::hash(arg).to_vec())
-                        }
-                    }
-                    None => Some(vec![]),
-                };
-                install_code.arg_hash = arg_hash;
-            }
-        }
-    }
-
     /// Creates a new neuron or refreshes the stake of an existing
     /// neuron from a ledger account.
     ///