#[pallet::call] doesn't handle feature gates correctly

[gupnik]

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Description of bug

The current macro expansion for #[pallet::call] doesn't handle the feature gates correctly, causing an error.

Steps to reproduce

For example, while trying to feature gate do_task in #1343 like so:

#[cfg(feature = "experimental")]
#[pallet::call_index(8)]
#[pallet::weight(task.weight())]
pub fn do_task(origin: OriginFor<T>, task: T::RuntimeTask) -> DispatchResultWithPostInfo {
...
}

the generated call enum still has do_task without the respective feature gate.