refactor(utils): remove unnecessary drivable area generation

[satoshi-ota]

Description

🤖 Generated by Copilot at 79ca700

Refactor getReferencePath function in utils.cpp to remove unnecessary computation of drivable area. This change is part of migrating the behavior path planner to the new architecture.

Related links

Nothing.

Tests performed

• PASS TIER IV INTERNAL SCENARIOS

Notes for reviewers

Nothing.

Interface changes

Nothing.

Effects on system behavior

Nothing.

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.
• The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• The PR follows the pull request guidelines.
• The PR has been properly tested.
• The PR has been reviewed by the code owners.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• There are no open discussions or they are tracked via tickets.
• The PR is ready for merge.

After all checkboxes are checked, anyone who has write access can merge the PR.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (48e10b5) 14.87% compared to head (228e43e) 14.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4291      +/-   ##
==========================================
- Coverage   14.87%   14.87%   -0.01%     
==========================================
  Files        1519     1519              
  Lines      105031   105029       -2     
  Branches    32010    32009       -1     
==========================================
- Hits        15628    15627       -1     
  Misses      72331    72331              
+ Partials    17072    17071       -1     

Flag	Coverage Δ		*Carryforward flag
differential	13.21% <ø> (?)
total	14.87% <ø> (-0.01%)	⬇️	Carriedforward from 48e10b5

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

Files Changed	Coverage Δ
planning/behavior_path_planner/src/utils/utils.cpp	15.61% <ø> (-0.05%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[zulfaqar-azmi-t4]

LGTM!

Additional note @satoshi-ota :
It was previously reported that start_planner's scenario has failed.
But at the moment, I couldn't reproduce them locally. And evaluator doesn't show any failure anymore. So I think this PR can be merged.

In case there's failure in evaluator tomorrow, I believe we dont need to revert this PR, but instead we should fix start planner's bug.