Remove redundant casts

[pjonsson]

Reason for this pull request

Remove redundant casts and configure MyPy to warn about them in the future.

Proposed changes

• Closes #xxxx

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

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📚 Documentation preview 📚: https://datacube-core--1758.org.readthedocs.build/en/1758/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.15%. Comparing base (1d5c302) to head (cc83fd5).
Report is 461 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1758   +/-   ##
========================================
  Coverage    85.15%   85.15%           
========================================
  Files          149      149           
  Lines        15525    15525           
========================================
  Hits         13221    13221           
  Misses        2304     2304           

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