fix(diagnostic_graph_aggregator): remove unused message alias

[isamu-takagi]

Description

Remove unused message alias.

Related links

tier4/tier4_autoware_msgs#145

How was this PR tested?

Check if the build passes.

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.17%. Comparing base (1e1d2fc) to head (e183449).
Report is 416 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8722      +/-   ##
==========================================
+ Coverage   24.15%   24.17%   +0.02%     
==========================================
  Files        1400     1400              
  Lines      102276   102342      +66     
  Branches    38805    38805              
==========================================
+ Hits        24701    24742      +41     
- Misses      75067    75092      +25     
  Partials     2508     2508              

Flag	Coverage Δ		*Carryforward flag
differential	63.36% <ø> (?)
total	24.15% <ø> (ø)		Carriedforward from 1e1d2fc

*This pull request uses carry forward flags. Click here to find out more.

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity.

[yukkysaito]

LGTM