util/diesel: Remove obsolete prelude module and Conn trait

[Turbo87]

These are both not needed anymore now that we use diesel_async::{AsyncPgConnection, RunQueryDsl} everywhere.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.51%. Comparing base (dfc30d0) to head (c67b728).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10056      +/-   ##
==========================================
- Coverage   89.51%   89.51%   -0.01%     
==========================================
  Files         295      295              
  Lines       31213    31211       -2     
==========================================
- Hits        27941    27939       -2     
  Misses       3272     3272              

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