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.

Github-Pull: #11100
Rebased-From: ba206d2c63

test/functional/mining.py

@@ -38,7 +38,6 @@ class MiningTest(BitcoinTestFramework):
         mining_info = node.getmininginfo()
         assert_equal(mining_info['blocks'], 200)
         assert_equal(mining_info['chain'], 'regtest')
-        assert_equal(mining_info['currentblocksize'], 0)
         assert_equal(mining_info['currentblocktx'], 0)
         assert_equal(mining_info['currentblockweight'], 0)
         assert_equal(mining_info['difficulty'], Decimal('4.656542373906925E-10'))