Remove `pallet::getter` usage from `pallet-nft-fractionalization` (#7124)

Description
Part of #3326
As per title, `pallet::getter` usage has been removed from
`pallet-nft-fractionalization`.

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/pr_7124.prdoc

+title: Remove pallet::getter from pallet-nft-fractionalization
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR removes all pallet::getter occurrences from pallet-nft-fractionalization and replaces them with explicit implementations.
+
+crates:
+  - name: pallet-nft-fractionalization
+    bump: major

substrate/frame/nft-fractionalization/src/lib.rs

@@ -159,7 +159,6 @@ pub mod pallet {
 
 	/// Keeps track of the corresponding NFT ID, asset ID and amount minted.
 	#[pallet::storage]
-	#[pallet::getter(fn nft_to_asset)]
 	pub type NftToAsset<T: Config> = StorageMap<
 		_,
 		Blake2_128Concat,
@@ -338,6 +337,13 @@ pub mod pallet {
 			T::PalletId::get().into_account_truncating()
 		}
 
+		/// Keeps track of the corresponding NFT ID, asset ID and amount minted.
+		pub fn nft_to_asset(
+			key: (T::NftCollectionId, T::NftId),
+		) -> Option<Details<AssetIdOf<T>, AssetBalanceOf<T>, DepositOf<T>, T::AccountId>> {
+			NftToAsset::<T>::get(key)
+		}
+
 		/// Prevent further transferring of NFT.
 		fn do_lock_nft(nft_collection_id: T::NftCollectionId, nft_id: T::NftId) -> DispatchResult {
 			T::Nfts::disable_transfer(&nft_collection_id, &nft_id)