refactor(front): use no column for pk of simple agg

[lmatz]

What's changed and what's your intention?

Surprised that nothing breaks down in the backend... 😲

Checklist

- [ ] I have written necessary docs and comments
- [ ] I have added necessary unit tests and integration tests

• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

closes #3120

[codecov]

Codecov Report

Merging #3185 (8371a6b) into main (e7c8e95) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3185      +/-   ##
==========================================
- Coverage   73.60%   73.60%   -0.01%     
==========================================
  Files         744      744              
  Lines      102025   102025              
==========================================
- Hits        75092    75091       -1     
- Misses      26933    26934       +1     

Flag	Coverage Δ
rust	73.60% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
...rc/frontend/src/optimizer/plan_node/logical_agg.rs	97.86% <100.00%> (ø)
src/common/src/types/ordered_float.rs	24.70% <0.00%> (-0.20%)	⬇️

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[skyzh]

That's cool 🤤🤤🤤