The fallback introduced in dash#5607 for spent UTXOs also works for
unspent UTXOs, no reason to leave it as fallback when it can be made
primary.
Also, if we did want to keep GetUTXOCoin around, it would need access
to CChainState due to the refactoring due in the next commit, which is
possible in its RPC invocations but isn't so readily available in Qt
code, where it is also called. The fallback code has the benefit of not
relying on CChainState.
