feat: Remove sessions datasets, storages, APIs and message processors

[lynnagara]

[hubertdeng123]

We've begun writing to the metrics backend for self-hosted as of 23.9.1 this month but want to give people a bit more time before a cutover. Let's plan to get this merged in November and say goodbye to sessions once and for all

[hubertdeng123]

@lynnagara we can revisit this now that sessions is gone

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5f605d1) 90.13% compared to head (5de097a) 89.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4785      +/-   ##
==========================================
- Coverage   90.13%   89.91%   -0.23%     
==========================================
  Files         891      885       -6     
  Lines       43395    43010     -385     
  Branches      288      288              
==========================================
- Hits        39116    38674     -442     
- Misses       4237     4294      +57     
  Partials       42       42              

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[mwarkentin]

Back from the dead? 🧟