[miner] always assume we can create witness blocks

Given the low possibility of a reorg reverting the segwit soft fork,
there is no need to check whether segwit is active here. Also,
TestBlockValidity is run on the block template after it has been
created.

src/node/miner.h

@@ -132,7 +132,6 @@ private:
     std::unique_ptr<CBlockTemplate> pblocktemplate;
 
     // Configuration parameters for the block size
-    bool fIncludeWitness;
     unsigned int nBlockMaxWeight;
     CFeeRate blockMinFeeRate;