Remove task arguments

[heyman]

This PR removes args and kwargs arguments from the internal TaskSet.schedule_task and TaskSet.execute_task methods.

Passing arguments to tasks was an old legacy feature that was never documented, and never used internally (except in tests). I think removing this feature in the name of KISS makes sense, since the feature aren't used internally, even though the methods are internal.

This will however be a breaking change for people who have looked at the Locust source and chosen to use this feature.

[codecov]

Codecov Report

Merging #1370 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1370      +/-   ##
==========================================
- Coverage   80.83%   80.76%   -0.08%     
==========================================
  Files          26       26              
  Lines        2265     2261       -4     
  Branches      345      345              
==========================================
- Hits         1831     1826       -5     
- Misses        342      343       +1     
  Partials       92       92              

Impacted Files	Coverage Δ
locust/user/task.py	96.55% <100.00%> (-0.77%)	⬇️
locust/runners.py	80.62% <0.00%> (ø)

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[cyberw]

LGTM. That code always looked strange to me :)