pallet-grandpa: Remove `GRANDPA_AUTHORITIES_KEY` (#2181)

Remove the `GRANDPA_AUTHORITIES_KEY` key and its usage. Apparently this
was used in the early days to communicate the grandpa authorities to the
node. However, we have now a runtime api that does this for us. So, this
pull request is moving from the custom managed storage item to a FRAME
managed storage item.

This pr also includes a migration for doing the switch on a running
chain.

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Co-authored-by: Davide Galassi <davxy@datawok.net>