[Enhancement] Discard deprecated lmdb dataset format and only support img+label now

[gaotongxiao]

No description provided.

[codecov]

Codecov Report

Base: 85.79% // Head: 85.70% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (4d4dc34) compared to base (b64565c).
Patch coverage: 86.20% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #1681      +/-   ##
===========================================
- Coverage    85.79%   85.70%   -0.10%     
===========================================
  Files          173      173              
  Lines        10919    10863      -56     
  Branches      1529     1519      -10     
===========================================
- Hits          9368     9310      -58     
- Misses        1285     1288       +3     
+ Partials       266      265       -1     

Flag	Coverage Δ
unittests	85.70% <86.20%> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
mmocr/datasets/transforms/loading.py	89.51% <ø> (-1.80%)	⬇️
mmocr/datasets/recog_lmdb_dataset.py	82.00% <86.20%> (-10.31%)	⬇️
...r/models/kie/postprocessors/sdmgr_postprocessor.py	91.17% <0.00%> (-2.95%)	⬇️
mmocr/datasets/transforms/textdet_transforms.py	91.76% <0.00%> (-0.29%)	⬇️
mmocr/datasets/transforms/textrecog_transforms.py	97.33% <0.00%> (+1.18%)	⬆️

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