coinstats: Move GetUTXOStats to rpc/blockchain

rpc/blockchain.cpp is now the only user of the vestigial
GetUTXOStats(...). And since GetUTXOStats(...)'s special fallback logic
was only really relevant/meant for rpc/blockchain.cpp, we can just move
it there.

src/Makefile.am

@@ -192,7 +192,6 @@ BITCOIN_CORE_H = \
   node/caches.h \
   node/chainstate.h \
   node/coin.h \
-  node/coinstats.h \
   node/context.h \
   node/miner.h \
   node/minisketchwrapper.h \
@@ -367,7 +366,6 @@ libbitcoin_node_a_SOURCES = \
   node/caches.cpp \
   node/chainstate.cpp \
   node/coin.cpp \
-  node/coinstats.cpp \
   node/context.cpp \
   node/interfaces.cpp \
   node/miner.cpp \