chore!: remove unused KeyPowerReduction var

[colin-axner]

Description

Closes: #XXXX

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Author Checklist

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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
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[codecov]

Codecov Report

Merging #10248 (f46934b) into master (e9390df) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10248   +/-   ##
=======================================
  Coverage   63.65%   63.65%           
=======================================
  Files         573      573           
  Lines       53761    53761           
=======================================
+ Hits        34221    34222    +1     
+ Misses      17591    17590    -1     
  Partials     1949     1949           

Impacted Files	Coverage Δ
x/staking/types/params.go	37.96% <ø> (ø)
crypto/keys/internal/ecdsa/privkey.go	84.21% <0.00%> (+1.75%)	⬆️

[amaury1093]

oh yeah, thanks. Surely some leftovers from #9495