refactor(utils): remove unnecessary variable

[satoshi-ota]

Description

the current_lanes is not used and unnecessary. I remove it.

Tests performed

Psim&Evalautor

Effects on system behavior

Psim

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e7f8c12) 14.07% compared to head (c70d7f7) 14.07%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3562   +/-   ##
=======================================
  Coverage   14.07%   14.07%           
=======================================
  Files        1398     1398           
  Lines       97963    97960    -3     
  Branches    29085    29083    -2     
=======================================
  Hits        13789    13789           
+ Misses      69501    69500    -1     
+ Partials    14673    14671    -2     

Flag	Coverage Δ		*Carryforward flag
differential	15.35% <ø> (?)
total	14.07% <ø> (+<0.01%)	⬆️	Carriedforward from e7f8c12

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

Impacted Files	Coverage Δ
planning/behavior_path_planner/src/utils/utils.cpp	16.42% <ø> (+0.04%)	⬆️

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