docs(subtensor): fix stale faucet() docstring + remove dead cfg branch

[ArtificialXai]

Closes #2591.

Context

The faucet() dispatchable in pallets/subtensor/src/macros/dispatches.rs carries this docstring:

/// Facility extrinsic for user to get taken from faucet
/// It is only available when pow-faucet feature enabled
/// Just deployed in testnet and devnet for testing purpose

The last line is stale. The call is #[cfg(feature = \"pow-faucet\")]-gated, and the pow-faucet feature is only enabled in the local_builder Docker stage (Dockerfile L86). The public testnet and devnet runtimes are built without it, so the dispatchable is absent from the on-chain Call enum on both networks (per the metadata evidence in #2591). Downstream tooling that trusts the docstring builds a working PoW pipeline only to fail at submission time with a pallet-call-not-found error.

Changes

1. Docstring rewrite — replaces the stale last line with an explicit note that the call is only compiled into runtimes built with --features pow-faucet, that the public testnet/devnet images do not include it, and that it is intended for local development runtimes.

2. Dead-code removal inside the function body:

   // before
   pub fn faucet(...) -> DispatchResult {
       if cfg!(feature = \"pow-faucet\") {
           return Self::do_faucet(origin, block_number, nonce, work);
       }
       Err(Error::<T>::FaucetDisabled.into())
   }

   The function is already #[cfg(feature = \"pow-faucet\")] at the item level, so cfg!(feature = \"pow-faucet\") inside the body is unconditionally true and the Err arm is unreachable. Simplified to a direct Self::do_faucet(...) call.

   FaucetDisabled is left in Error<T> for backward compatibility — it is also referenced by a commented-out guard in subnets/registration.rs and removing it could break downstream error-decoding clients.

Behavioral impact

None. Builds without pow-faucet don't see the dispatchable; builds with it exercise the same Self::do_faucet(...) path that was always reached. #[pallet::call_index(60)] and the weight attribute are preserved, so on-chain extrinsic compatibility is unchanged.

Out of scope

Option (b) from #2591 (enabling pow-faucet on the testnet Docker build) is not addressed here — that's a policy call about whether the public testnet runtime should ship a dispatchable that mints TAO, and the filer explicitly said either fix is acceptable. Happy to follow up in a separate PR if maintainers want to restore the historical btcli wallet faucet UX against testnet.