Remove asv benchmarking

[NicolaCourtier]

Description

Remove the files and comments relating to the Airspeed Velocity benchmarking, which is no longer working or needed.

Fixes #917

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[agriyakhetarpal]

I think we can also remove multiprocessing_bench.py from the repo root while we are at it and resolve #916, since it was also there for benchmarking-related reasons. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.12%. Comparing base (4628a65) to head (1bfbc53).
⚠️ Report is 64 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #960      +/-   ##
===========================================
+ Coverage    89.41%   92.12%   +2.71%     
===========================================
  Files           63       74      +11     
  Lines         4857     5793     +936     
===========================================
+ Hits          4343     5337     +994     
+ Misses         514      456      -58     

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