revert paritytech/polkadot#6577 & related changes (#3108)

Moves `pallet_mmr` back behind `pallet_session` to address
polkadot-fellows/runtimes#160.

Opening draft for CI - should be merged or closed depending on outcome
of w3f/polkadot-spec#718.

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Co-authored-by: Adrian Catangiu <adrian@parity.io>