Remove no longer used fields on FlowStart and Msg models

[rowanseymour]

Requires nyaruka/mailroom#626 and nyaruka/rp-archiver#116

DROP TABLE "flows_flowstart_calls";
ALTER TABLE "flows_flowstart" DROP COLUMN "parent_summary";
ALTER TABLE "flows_flowstart" DROP COLUMN "session_history";
ALTER TABLE "msgs_msg" DROP COLUMN "metadata";

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f24cebe) to head (313f4ab).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #6088   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          557       559    +2     
  Lines        25414     25414           
=========================================
  Hits         25414     25414           

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