refacto: 🔧 postprocessing with rotated boxes

[charlesmindee]

Following #632 and #551, this PR removes deprecated flags and deprecated page-rotation steps that are no longer required since we are working with rotated boxes when assume_straight_pages is False.

Any feedback is welcome!!

[codecov]

Codecov Report

Merging #641 (9b4aa76) into main (74ff9ff) will decrease coverage by 0.03%.
The diff coverage is 97.91%.

@@            Coverage Diff             @@
##             main     #641      +/-   ##
==========================================
- Coverage   96.09%   96.06%   -0.04%     
==========================================
  Files         112      112              
  Lines        4357     4349       -8     
==========================================
- Hits         4187     4178       -9     
- Misses        170      171       +1     

Flag	Coverage Δ
unittests	96.06% <97.91%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/detection/linknet/base.py	88.00% <90.90%> (ø)
doctr/models/detection/core.py	97.29% <100.00%> (-0.33%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.92% <100.00%> (+0.10%)	⬆️
...s/detection/differentiable_binarization/pytorch.py	98.30% <100.00%> (ø)
...etection/differentiable_binarization/tensorflow.py	97.36% <100.00%> (ø)
doctr/models/detection/linknet/pytorch.py	96.26% <100.00%> (ø)
doctr/models/detection/linknet/tensorflow.py	96.51% <100.00%> (ø)
doctr/models/detection/predictor/pytorch.py	94.44% <100.00%> (ø)
doctr/models/detection/predictor/tensorflow.py	100.00% <100.00%> (ø)
doctr/models/predictor/base.py	100.00% <100.00%> (ø)
... and 3 more

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[fg-mindee]

Thanks a lot Charles 🙏 Looks good to me!