Resolve the checkpoints <-> validation CD.

This commit resolves the checkpoints -> validation -> checkpoints
cirular dependency by moving
`CheckPoints::GetLastCheckpoint(const CCheckpointData& data)` from
`checkpoints.cpp` to `validation.cpp`.

src/qt/clientmodel.cpp

@@ -11,7 +11,6 @@
 
 #include <chain.h>
 #include <chainparams.h>
-#include <checkpoints.h>
 #include <clientversion.h>
 #include <interfaces/handler.h>
 #include <interfaces/node.h>