Tablet throttler: remove LowPriority logic

[shlomi-noach]

Description

This is a cleanup PR for a functionality we've never used. While these changes are extracted from #15988, this PR is against main and not as part of the multi-metrics PR series.

The "low priority" mechanism is a relic we inherited from freno, where production client apps competed with background apps over the throttler. We have never used this in vitess, to date, and I therefore wish to remove it. It only adds more boilerplate, more considerations, and as a matter in fact I spent the better half of a day chasing down a recurring CI failure that was caused due to a (bug? misconfiguration?) related to prioritization.

In vitess, all existing throttler clients have the same priority and nothing externally to vitess gets any precedence.

The priority flag & usage were never documented in the first place and users were not instructed to use it. Priorities were mentioned in the docs but without any actionable guidance.

Moreover, this functionality did not enjoy any specific unit testing.

Doc updates: #16013

Related Issue(s)

#15624

Checklist

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Deployment Notes

[vitess-bot]

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[shlomi-noach]

Users can still supply p param, but it will have no effect.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.26%. Comparing base (d95ce48) to head (80a0977).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16013      +/-   ##
==========================================
- Coverage   68.27%   68.26%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      197319   197298      -21     
==========================================
- Hits       134714   134692      -22     
- Misses      62605    62606       +1     

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