mark pallet-asset-rate optional in polkadot-runtime-common (#2187)
Part of #2186 The only usage of pallet-asset-rate is guarded by `runtime-benchmarks` feature. I don't want ORML to be forced to include this pallet in deps for no good reason.
Part of #2186 The only usage of pallet-asset-rate is guarded by `runtime-benchmarks` feature. I don't want ORML to be forced to include this pallet in deps for no good reason.