Require fn token in decl_storage get (#5717)

* Require `fn` token in `decl_storage` `get`

The `fn` token was already for quite some time as an optional parameter.
It was introduced to make these functions better findable. This pr makes
the `fn` token required.

* Remove `GetterNoFnkeyword`

substrate/frame/support/src/lib.rs

@@ -271,8 +271,6 @@ mod tests {
 				map hasher(identity) T::BlockNumber => T::BlockNumber;
 			pub GenericData2 get(fn generic_data2):
 				map hasher(blake2_128_concat) T::BlockNumber => Option<T::BlockNumber>;
-			pub GetterNoFnKeyword get(no_fn): Option<u32>;
-
 			pub DataDM config(test_config) build(|_| vec![(15u32, 16u32, 42u64)]):
 				double_map hasher(twox_64_concat) u32, hasher(blake2_128_concat) u32 => u64;
 			pub GenericDataDM:
@@ -558,15 +556,6 @@ mod tests {
 					),
 					documentation: DecodeDifferent::Encode(&[]),
 				},
-				StorageEntryMetadata {
-					name: DecodeDifferent::Encode("GetterNoFnKeyword"),
-					modifier: StorageEntryModifier::Optional,
-					ty: StorageEntryType::Plain(DecodeDifferent::Encode("u32")),
-					default: DecodeDifferent::Encode(
-						DefaultByteGetter(&__GetByteStructGetterNoFnKeyword(PhantomData::<Test>))
-					),
-					documentation: DecodeDifferent::Encode(&[]),
-				},
 				StorageEntryMetadata {
 					name: DecodeDifferent::Encode("DataDM"),
 					modifier: StorageEntryModifier::Default,