investigate errors when skipping xcm benchmarks for collectives-polkadot

[brenzi]

While fixing XCM benchmarking for encointer in #440, I got an error while running xcm::generic benchmarks:
https://gist.github.com/brenzi/bb6050f9cb00d34f279cf1f4e5b6e4ba

I did

~/bin/polkadot-parachain-v1.14.0-runtime-benchmarks benchmark pallet --chain ./encointer-kusama-chain-spec.json --pallet=pallet-xcm-benchmarks::generic --extrinsic="*"

and got Error executing and verifying runtime benchmark: Benchmark not found for this pallet.

This error should be ignored because the benchmark is skipped for our runtime, as this functionality isn't provided by encointer (exactly like people chain for example).

I relalized that a check needs to be removed and that the coretime chain still has this check implemented. I guess it should be removed too?

runtimes/system-parachains/collectives/collectives-polkadot/src/lib.rs

Line 1164 in 790ef91

if batches.is_empty() { return Err("Benchmark not found for this pallet.".into()) }