chore(sns): Remove unnecessary comment (#2750)

This comment no longer applies now that we've migrated to ic-cdk. SNS
Swap and SNS Root did not have analagous comments in their canister.rs

rs/sns/governance/canister/canister.rs

@@ -1,14 +1,3 @@
-// Note on `candid_method`: each canister method should have a function
-// annotated with `#[candid_method]` that has the arguments and return type
-// expected by the canister method, to be able to generate `governance.did`
-// automatically.
-//
-// This often means we need a function with `#[export_name = "canister_query
-// my_method"]` that doesn't take arguments and doesn't return anything (per IC
-// spec), then another function with the actual method arguments and return
-// type, annotated with `#[candid_method(query/update)]` to be able to generate
-// the did definition of the method.
-
 use async_trait::async_trait;
 use ic_base_types::{CanisterId, PrincipalId};
 use ic_canister_log::log;