Remove all remaining traces of nbqa from the repo

[drbenvincent]

• In Fix up ruff pre-commit checks to operate on notebooks #352 I forgot to nuke all mention of nbqa from the repo
• This PR does that, removing it from pyproject.toml and the Makefile

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.98%. Comparing base (e092650) to head (013b06b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #353   +/-   ##
=======================================
  Coverage   79.98%   79.98%           
=======================================
  Files          21       21           
  Lines        1634     1634           
=======================================
  Hits         1307     1307           
  Misses        327      327           

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[juanitorduz]

+1