Wdpost worker: Reduce challenge confidence to 1 epoch

[arajasek]

This confidence was introduced in #5764 as a stopgap until #3613 was fixed, which happened in the chocolate upgrade.

It is now strictly unnecessary and the 5 minutes can be spent on PoSt computation. We could kill it entirely, or leave it at 1 epoch.

[codecov]

Codecov Report

Merging #7572 (6455b0e) into master (d9fd1d9) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7572      +/-   ##
==========================================
- Coverage   39.89%   39.77%   -0.13%     
==========================================
  Files         633      633              
  Lines       67098    67098              
==========================================
- Hits        26771    26685      -86     
- Misses      35731    35821      +90     
+ Partials     4596     4592       -4     

Impacted Files	Coverage Δ
storage/wdpost_changehandler.go	98.11% <ø> (-0.48%)	⬇️
chain/actors/builtin/market/v5.go	23.38% <0.00%> (-16.13%)	⬇️
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
markets/loggers/loggers.go	89.28% <0.00%> (-10.72%)	⬇️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
storage/wdpost_sched.go	81.18% <0.00%> (-5.95%)	⬇️
markets/storageadapter/ondealsectorcommitted.go	77.33% <0.00%> (-4.00%)	⬇️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
chain/actors/builtin/miner/v5.go	35.69% <0.00%> (-2.47%)	⬇️
extern/sector-storage/stores/local.go	57.30% <0.00%> (-1.97%)	⬇️
... and 14 more

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