memtx: refactor story cleanup on space delete

When a space is deleted, all transactions need to be aborted and all their
stories need to be removed immediately out of order: currently we
artificially rollback statements — instead call this statement
removal to logically distinguish it from rollback. It differs in the sense
that the whole space's tuple history is teared down instead — no more
transaction managing is going to be done as opposed to rollback of an
individual transaction.

Needed for #7343

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