Remove custom panic handler once `sp-io` is upgraded

[dmitry-markin]

PR #17 introduces a custom panic handler to fix compilation: https://github.com/paritytech/polkadot-bulletin-chain/pull/17/files#diff-3da9181f250965f0e602cfe4b676b4ad4558540078f3169bc71f0e8330cb9af7R6-R7

This custom panic handler should be removed once sp-io is upgraded (and the panic handler shipped with it builds with the latest rustc).

[bkontur]

@dmitry-markin this is fixed, right?