Remove pre-generated annotation files for ctw1500

[gaotongxiao]

New users using provided instances_training.json and instances_test.json may not run ctw1500_converter.py and skip necessary proprocessing steps, leading to errors in training.

The tutorial has now been changed to ensure users would preprocess ctw1500 correctly.

[codecov]

Codecov Report

Merging #268 (68d73cd) into main (b1741de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #268   +/-   ##
=======================================
  Coverage   85.87%   85.87%           
=======================================
  Files         132      132           
  Lines        8802     8802           
  Branches     1231     1231           
=======================================
  Hits         7559     7559           
  Misses        941      941           
  Partials      302      302           

Flag	Coverage Δ
unittests	85.87% <ø> (ø)

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