Follow-up from "add UnpackRawEvent"

The following items from !1102 (merged) should be addressed:

• UnpackRawEvent should be included in the canonical sequences for the decoding the RawEvent
• The subsystem decoders should be adapted to use LHCb::RawBank::View as input from the TES instead of RawEvent
• The (mutable!) code in RawEvent which creates the map from type -> LHCb::RawBank::View should be moved to UnpackRawEvent, as that is (probably) the only place this is needed, as it creates LHCb::RawBank::View per type
• The mutable eventMap can then be removed from RawEvent.

See also LBCORE-1742