[pallet-broker] add extrinsic to remove an assignment (#7080)

# Description

#6929 requests more extrinsics for "managing the network's coretime
allocations without needing to dabble with migration+runtime upgrade or
set/kill storage patterns"

This pull request implements the remove_assignment() extrinsic.


## Integration

Downstream projects need to benchmark the weight for the
remove_assignment() extrinsic.

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Co-authored-by: Jonathan Brown <jbrown@acuity.network>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>