Clean up subnet identities unused storage

[l0r1s]

The subtensor pallet lib.rs contains no less than 5 identities storage items:

    /// --- MAP ( coldkey ) --> identity. (DEPRECATED for V2)
    #[pallet::storage]
    pub type Identities<T: Config> =
        StorageMap<_, Blake2_128Concat, T::AccountId, ChainIdentityOf, OptionQuery>;

    /// --- MAP ( coldkey ) --> identity
    #[pallet::storage]
    pub type IdentitiesV2<T: Config> =
        StorageMap<_, Blake2_128Concat, T::AccountId, ChainIdentityOfV2, OptionQuery>;

    /// --- MAP ( netuid ) --> identity. (DEPRECATED for V2)
    #[pallet::storage]
    pub type SubnetIdentities<T: Config> =
        StorageMap<_, Blake2_128Concat, NetUid, SubnetIdentityOf, OptionQuery>;

    /// --- MAP ( netuid ) --> identityV2 (DEPRECATED for V3)
    #[pallet::storage]
    pub type SubnetIdentitiesV2<T: Config> =
        StorageMap<_, Blake2_128Concat, NetUid, SubnetIdentityOfV2, OptionQuery>;

    /// --- MAP ( netuid ) --> SubnetIdentityOfV3
    #[pallet::storage]
    pub type SubnetIdentitiesV3<T: Config> =
        StorageMap<_, Blake2_128Concat, NetUid, SubnetIdentityOfV3, OptionQuery>;

This needs to be migrated if needed and cleaned up.