Revert "feat(issue-consumer): make batched parallel mode default"

[vartec]

Reverts #71572

Reverting as mitigation for INC-811
We need to figure out better way to scale for CPU-bound workload.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.02%. Comparing base (27f0f97) to head (4e84a4a).
Report is 739 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #73554      +/-   ##
==========================================
- Coverage   79.85%   78.02%   -1.84%     
==========================================
  Files        6811     6638     -173     
  Lines      306482   296882    -9600     
  Branches    51122    51122              
==========================================
- Hits       244748   231641   -13107     
+ Misses      61351    58968    -2383     
- Partials      383     6273    +5890     

Files	Coverage Δ
src/sentry/consumers/__init__.py	78.94% <ø> (-4.64%)	⬇️

... and 1756 files with indirect coverage changes

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀