util: Simplify path argument for CBlockTreeDB ctor
util: Simplify path argument for CBlockTreeDB ctor

This commit does not change behavior as GetBlocksDir() with unset 
"-blocksdir" returns the same path.

DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin