chore!: remove unused KeyPowerReduction var (cosmos#10248)

* remove unused KeyPowerReduction var from x/staking

* add changelog

CHANGELOG.md

@@ -85,6 +85,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * [\#9432](https://github.com/cosmos/cosmos-sdk/pull/9432) `ConsensusParamsKeyTable` moved from `params/keeper` to `params/types`
 * [\#9576](https://github.com/cosmos/cosmos-sdk/pull/9576) Add debug error message to `sdkerrors.QueryResult` when enabled
 * [\#9650](https://github.com/cosmos/cosmos-sdk/pull/9650) Removed deprecated message handler implementation from the SDK modules.
+* [\#10248](https://github.com/cosmos/cosmos-sdk/pull/10248) Remove unused `KeyPowerReduction` variable from x/staking types.
 * (x/bank) [\#9832] (https://github.com/cosmos/cosmos-sdk/pull/9832) `AddressFromBalancesStore` renamed to `AddressAndDenomFromBalancesStore`.
 * (tests) [\#9938](https://github.com/cosmos/cosmos-sdk/pull/9938) `simapp.Setup` accepts additional `testing.T` argument.
 * (baseapp) [\#9920](https://github.com/cosmos/cosmos-sdk/pull/9920) BaseApp `{Check,Deliver,Simulate}Tx` methods are now replaced by a middleware stack.

x/staking/types/params.go

@@ -38,7 +38,6 @@ var (
 	KeyMaxEntries        = []byte("MaxEntries")
 	KeyBondDenom         = []byte("BondDenom")
 	KeyHistoricalEntries = []byte("HistoricalEntries")
-	KeyPowerReduction    = []byte("PowerReduction")
 )
 
 var _ paramtypes.ParamSet = (*Params)(nil)