test: remove fSkipProofOfWork

Not used, and REGTEST already allows creating blocks at
the lowerst possible difficulty.

src/chainparams.h

@@ -60,8 +60,6 @@ public:
     bool DefaultCheckMemPool() const { return fDefaultCheckMemPool; }
     /** Allow mining of a min-difficulty block */
     bool AllowMinDifficultyBlocks() const { return fAllowMinDifficultyBlocks; }
-    /** Skip proof-of-work check: allow mining of any difficulty block */
-    bool SkipProofOfWorkCheck() const { return fSkipProofOfWorkCheck; }
     /** Make standard checks */
     bool RequireStandard() const { return fRequireStandard; }
     int64_t TargetTimespan() const { return nTargetTimespan; }
@@ -104,7 +102,6 @@ protected:
     bool fAllowMinDifficultyBlocks;
     bool fRequireStandard;
     bool fMineBlocksOnDemand;
-    bool fSkipProofOfWorkCheck;
     bool fTestnetToBeDeprecatedFieldRPC;
 };