Deprecate confusing blockmaxsize, fix getmininginfo output

* This removes block-size-limiting code in favor of GBT clients
  doing the limiting themselves (if at all).
* -blockmaxsize is deprecated and only used to calculate an implied
  blockmaxweight, addressing confusion from multiple users.
* getmininginfo's currentblocksize return value was returning
  garbage values, and has been removed, also removing a
  GetSerializeSize call in some block generation inner loops and
  potentially addressing some performance edge cases.

src/miner.h

@@ -139,13 +139,11 @@ private:
 
     // Configuration parameters for the block size
     bool fIncludeWitness;
-    unsigned int nBlockMaxWeight, nBlockMaxSize;
-    bool fNeedSizeAccounting;
+    unsigned int nBlockMaxWeight;
     CFeeRate blockMinFeeRate;
 
     // Information on the current status of the block
     uint64_t nBlockWeight;
-    uint64_t nBlockSize;
     uint64_t nBlockTx;
     uint64_t nBlockSigOpsCost;
     CAmount nFees;