refactor(frontend): remove unused agg aliases

[xiangjinwu]

What's changed and what's your intention?

This aliases field in LogicalAgg has always been None (except in unit tests) and will not be used in the future (#1433).

To reiterate: aliases will be user-facing and handled at bound sql level but not in plan nodes, while field names are for internal debugging only and can follow whatever rules, possibly not matching aliases.

Checklist

- [ ] I have written necessary docs and comments
- [ ] I have added necessary unit tests and integration tests (Refactor does not break existing tests.)

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #2115 (3e35685) into main (8b6bd87) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2115      +/-   ##
==========================================
- Coverage   70.66%   70.64%   -0.02%     
==========================================
  Files         636      636              
  Lines       80752    80703      -49     
==========================================
- Hits        57063    57014      -49     
  Misses      23689    23689              

Flag	Coverage Δ
rust	70.64% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rc/frontend/src/optimizer/plan_node/logical_agg.rs	97.51% <100.00%> (-0.19%)	⬇️
.../frontend/src/optimizer/rule/unnest_agg_for_loj.rs	98.73% <100.00%> (ø)
src/frontend/src/planner/select.rs	89.43% <100.00%> (-0.08%)	⬇️
src/meta/src/barrier/mod.rs	69.00% <0.00%> (-0.34%)	⬇️
src/common/src/types/ordered_float.rs	24.30% <0.00%> (+0.19%)	⬆️

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[likg227]

LGTM