refactor(rust): Remove unused IR::Reduce node

[orlp]

This was intended as a special case for select which only returns a single row but physical planners can already inspect the expressions in a select to determine whether this is the case regardless.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.94%. Comparing base (2c9e503) to head (813995a).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20392      +/-   ##
==========================================
+ Coverage   78.91%   78.94%   +0.03%     
==========================================
  Files        1567     1560       -7     
  Lines      219928   219635     -293     
  Branches     2465     2471       +6     
==========================================
- Hits       173549   173389     -160     
+ Misses      45811    45678     -133     
  Partials      568      568              

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