refactor(trajectory_followers): cosmetic changes

[TakaHoribe]

Description

Minor refactors

• remove unused code
• shrink namespace code

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[takayuki5168]

I approved in advance. This comment must be resolved though.

[codecov]

Codecov Report

Base: 11.49% // Head: 12.12% // Increases project coverage by +0.62% 🎉

Coverage data is based on head (8ed2e95) compared to base (132c154).
Patch coverage: 67.74% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2620      +/-   ##
==========================================
+ Coverage   11.49%   12.12%   +0.62%     
==========================================
  Files        1271     1271              
  Lines       88413    89347     +934     
  Branches    23314    23909     +595     
==========================================
+ Hits        10163    10829     +666     
- Misses      67480    67543      +63     
- Partials    10770    10975     +205     

Flag	Coverage Δ		*Carryforward flag
differential	47.44% <68.75%> (?)
total	11.49% <62.50%> (ø)		Carriedforward from 132c154

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

Impacted Files	Coverage Δ
..._controller/include/mpc_lateral_controller/mpc.hpp	44.82% <ø> (-1.84%)	⬇️
...pc_lateral_controller/qp_solver/qp_solver_osqp.hpp	0.00% <ø> (ø)
...l_controller/qp_solver/qp_solver_unconstr_fast.hpp	100.00% <ø> (ø)
...r/vehicle_model/vehicle_model_bicycle_dynamics.hpp	100.00% <ø> (ø)
...vehicle_model/vehicle_model_bicycle_kinematics.hpp	100.00% <ø> (ø)
...odel/vehicle_model_bicycle_kinematics_no_delay.hpp	100.00% <ø> (ø)
control/mpc_lateral_controller/src/interpolate.cpp	76.81% <0.00%> (+5.38%)	⬆️
control/mpc_lateral_controller/src/mpc.cpp	54.45% <0.00%> (+7.39%)	⬆️
..._lateral_controller/src/mpc_lateral_controller.cpp	43.66% <0.00%> (+8.56%)	⬆️
...trol/mpc_lateral_controller/src/mpc_trajectory.cpp	84.00% <ø> (+2.75%)	⬆️
... and 46 more

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[takayuki5168]

LGTM