sqlite->pyreport: stop serializing session_totals, just use null

[matt-codecov]

• delete SessionTotalsArray, remove from report shared#329
• remove session_totals field from report json worker#624
• remove session_totals field from report json codecov-api#752

in a report json, each file in the "files" key had a slot for a session_totals field. turns out the field was bugged and also unused, so we're deleting it.

in the pyreport->sqlite code, we always ignored that field and opted to recompute totals on demand later so that code doesn't need to be updated. the sqlite->pyreport code on the other hand gets a good deal simpler and probably faster.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (3fcba40) to head (3372f6b).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   98.62%   98.68%   +0.06%     
==========================================
  Files          17       17              
  Lines        7393     7315      -78     
==========================================
- Hits         7291     7219      -72     
+ Misses        102       96       -6     

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