refactor(ekf_localizer): remove the IDX struct and replace to the actual numbers

[IshitaTakeshi]

Description

The IDX struct doesn't really improve the readability, so replaced it with actual numbers.

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[codecov]

Codecov Report

Merging #1732 (11c43a8) into main (ae833f3) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1732      +/-   ##
==========================================
+ Coverage   10.28%   10.29%   +0.01%     
==========================================
  Files        1205     1203       -2     
  Lines       86798    86662     -136     
  Branches    20067    20067              
==========================================
  Hits         8926     8926              
+ Misses      68712    68576     -136     
  Partials     9160     9160              

Flag	Coverage Δ		*Carryforward flag
differential	0.00% <0.00%> (?)
total	10.28% <0.00%> (+0.01%)	⬆️	Carriedforward from 8f7898d

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

Impacted Files	Coverage Δ
..._localizer/include/ekf_localizer/ekf_localizer.hpp	0.00% <ø> (ø)
localization/ekf_localizer/src/ekf_localizer.cpp	0.00% <0.00%> (ø)
perception/tensorrt_yolo/src/nodelet.cpp	0.00% <0.00%> (ø)
perception/lidar_centerpoint/src/node.cpp	0.00% <0.00%> (ø)
perception/tensorrt_yolo/lib/src/trt_yolo.cpp	0.00% <0.00%> (ø)
...erception/tensorrt_yolo/lib/include/calibrator.hpp	0.00% <0.00%> (ø)
...erception/tensorrt_yolo/lib/include/cuda_utils.hpp	0.00% <0.00%> (ø)
...tion/traffic_light_classifier/utils/trt_common.cpp	0.00% <0.00%> (ø)
...on/lidar_apollo_instance_segmentation/src/node.cpp	0.00% <0.00%> (ø)
... and 29 more

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[YamatoAndo]

I think the code using the IDX is easier to understand which element it refers to.

[IshitaTakeshi]

I think it is better to this one after cleaning other parts of the code so I close this one