Final removal of `.accounting._mktinfo.Symbol`

[goodboy]

Lingering task(s) from #489 that we couldn't quite wrap up due to,

• the Symbol usage in .ui.order_mode:

  • OrderMode.load_unknown_dialog_from_msg()
    https://github.com/pikers/piker/blob/master/piker/ui/order_mode.py#L641
    • currently can't be swapped over easily to MktPair bc we don't
      relay enough info through the clearing sys msg set to reproduce
      via the new `.from_fqme() method..
    • probably best solution is to instead require all backend's to
      provide the MktPair msg directly as part of loading pre-existing
      live dialogs?
    • likely pertains to Unify EMS messaging set #514, Dark order and alert clearing tests #466, Dark order and alert persistence #463, order_mode further UX refinement ideas needing feedback #220

• deribit backend is still missing Asset usage in MktPairs and
  i only wasn't able to due this due to having no account set up to get
  the data APIs..

• there's a hacky shim-usage of MktPair for the fsp flumes in
  ui._fsp: https://github.com/pikers/piker/blob/rekt_pps/piker/ui/_fsp.py#L493

  • we should probably consider making a Cascade msg-type which
    describes IO for cascaded Flumes? Probably something to get into
    when we get back to formalizing that subsys XD

• Editting initial live order load fails? #502 seems to be related to this as well?