[Fix] Remove the legacy hack function for monocular dataset browsing

[Tai-Wang]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.
Remove the legacy hack function for monocular dataset browsing.

Modification

Please briefly describe what modification is made in this PR.
Only remove an unused function.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[Wuziyi616]

Sorry, this PR is not approved yet. Need to resolve conflict and unit test (I think there exists some import errors)?

[Wuziyi616]

There are some other imports like here, here, here and here

[codecov]

Codecov Report

Merging #794 (38f2ae7) into master (a8f4752) will increase coverage by 0.00%.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master     #794   +/-   ##
=======================================
  Coverage   48.94%   48.94%           
=======================================
  Files         208      210    +2     
  Lines       15884    15960   +76     
  Branches     2538     2547    +9     
=======================================
+ Hits         7774     7812   +38     
- Misses       7608     7646   +38     
  Partials      502      502           

Flag	Coverage Δ
unittests	48.94% <75.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet3d/models/detectors/single_stage_mono3d.py	16.66% <ø> (+0.18%)	⬆️
mmdet3d/core/bbox/structures/utils.py	79.16% <50.00%> (-17.98%)	⬇️
mmdet3d/apis/inference.py	43.21% <100.00%> (-0.85%)	⬇️
mmdet3d/datasets/nuscenes_mono_dataset.py	37.08% <100.00%> (-0.39%)	⬇️
mmdet3d/models/segmentors/encoder_decoder.py	20.94% <0.00%> (-0.70%)	⬇️
mmdet3d/datasets/pipelines/transforms_3d.py	90.39% <0.00%> (-0.48%)	⬇️
mmdet3d/models/losses/__init__.py	100.00% <0.00%> (ø)
mmdet3d/models/decode_heads/__init__.py	100.00% <0.00%> (ø)
mmdet3d/ops/group_points/group_points.py	31.18% <0.00%> (ø)
... and 6 more

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[Wuziyi616]

I think this PR is ready for merge @ZwwWayne