[backport] Remove cupy.array_equal, since it's not compatible with cuPy 7.8 (#6528)

[trivialfis]

No description provided.

[codecov-io]

Codecov Report

Merging #6535 (d5c5de5) into release_1.3.0 (bce7ca3) will decrease coverage by 0.12%.
The diff coverage is 0.00%.

@@                Coverage Diff                @@
##           release_1.3.0    #6535      +/-   ##
=================================================
- Coverage          79.99%   79.86%   -0.13%     
=================================================
  Files                 12       12              
  Lines               3494     3497       +3     
=================================================
- Hits                2795     2793       -2     
- Misses               699      704       +5     

Impacted Files	Coverage Δ
python-package/xgboost/sklearn.py	89.21% <0.00%> (-0.41%)	⬇️
python-package/xgboost/tracker.py	93.98% <0.00%> (-1.13%)	⬇️
python-package/xgboost/training.py	95.90% <0.00%> (-0.18%)	⬇️
python-package/xgboost/core.py	80.84% <0.00%> (+0.26%)	⬆️

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