refactor(behavior_velocity_planner): remove unused local interpolate function

[TakaHoribe]

Related Issue(required)

#430

Description(required)

• The LinearInterpolate class has been removed from interpolate.cpp.
• The rest of the functions have been moved to path_utilization.cpp and the interpolate.cpp/hpp files have been removed.

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[codecov]

Codecov Report

Merging #435 (9f35483) into tier4/proposal (9ce55c7) will decrease coverage by 4.00%.
The diff coverage is 0.00%.

@@                Coverage Diff                @@
##           tier4/proposal    #435      +/-   ##
=================================================
- Coverage           10.20%   6.19%   -4.01%     
=================================================
  Files                 705      71     -634     
  Lines               49427    4873   -44554     
  Branches             6413     678    -5735     
=================================================
- Hits                 5043     302    -4741     
+ Misses              40213    4428   -35785     
+ Partials             4171     143    -4028     

Impacted Files	Coverage Δ
...city_planner/src/scene_module/blind_spot/scene.cpp	0.00% <0.00%> (ø)
...c/scene_module/intersection/scene_intersection.cpp	0.00% <ø> (ø)
...ule/intersection/scene_merge_from_private_road.cpp	0.00% <ø> (ø)
...ity_planner/src/scene_module/intersection/util.cpp	0.00% <0.00%> (ø)
...module/no_stopping_area/scene_no_stopping_area.cpp	0.00% <ø> (ø)
...ene_module/occlusion_spot/occlusion_spot_utils.cpp	11.11% <ø> (+2.02%)	⬆️
...sion_spot/scene_occlusion_spot_in_private_road.cpp	0.00% <0.00%> (ø)
...usion_spot/scene_occlusion_spot_in_public_road.cpp	0.00% <0.00%> (ø)
...ocity_planner/src/utilization/path_utilization.cpp	0.00% <0.00%> (-26.67%)	⬇️
...ity_planner/test/src/test_occlusion_spot_utils.cpp	33.33% <ø> (-9.53%)	⬇️
... and 635 more

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[tkimura4]

LGTM