[Enhance] Remove useless param in 3D Seg Dataset classes

[Wuziyi616]

Remove label_weight because it's actually used in the loss function of Model class

[codecov]

Codecov Report

Merging #607 (af29209) into master (d5bddd2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #607      +/-   ##
==========================================
- Coverage   50.95%   50.92%   -0.03%     
==========================================
  Files         197      197              
  Lines       15076    15056      -20     
  Branches     2452     2444       -8     
==========================================
- Hits         7682     7668      -14     
+ Misses       6887     6884       -3     
+ Partials      507      504       -3     

Flag	Coverage Δ
unittests	50.92% <100.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
mmdet3d/datasets/custom_3d_seg.py	67.53% <100.00%> (+0.03%)	⬆️
mmdet3d/datasets/s3dis_dataset.py	94.66% <100.00%> (+3.65%)	⬆️
mmdet3d/datasets/scannet_dataset.py	87.85% <100.00%> (ø)

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