Use "original" type for BlockWeights

[notlesh]

What does it do?

Fixes try-runtime by ensuring that moonbase runtime declares the same type for BlockWeights as it did before #1576.

This is necessary because of how our impl_runtime_apis_plus_common macro uses the type:

            #[cfg(feature = "try-runtime")]
            impl frame_try_runtime::TryRuntime<Block> for Runtime {
                fn on_runtime_upgrade() -> (Weight, Weight) {
                    log::info!("try-runtime::on_runtime_upgrade()");
                    // NOTE: intentional expect: we don't want to propagate the error backwards,
                    // and want to have a backtrace here. If any of the pre/post migration checks
                    // fail, we shall stop right here and right now.
                    let weight = Executive::try_runtime_upgrade()
                        .expect("runtime upgrade logic *must* be infallible");
                    (weight, BlockWeights::get().max_block)
                }

                fn execute_block_no_check(block: Block) -> Weight {
                    Executive::execute_block_no_check(block)
                }
            }