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/procedural/src/storage/parse.rs

@@ -166,7 +166,7 @@ struct DeclStorageLine {
 
 #[derive(Parse, ToTokens, Debug)]
 struct DeclStorageGetterBody {
-	fn_keyword: Option<Token![fn]>,
+	fn_keyword: Token![fn],
 	ident: Ident,
 }