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`.

test/lint/lint-circular-dependencies.sh

@@ -10,7 +10,6 @@ export LC_ALL=C
 
 EXPECTED_CIRCULAR_DEPENDENCIES=(
     "chainparamsbase -> util/system -> chainparamsbase"
-    "checkpoints -> validation -> checkpoints"
     "index/txindex -> validation -> index/txindex"
     "policy/fees -> txmempool -> policy/fees"
     "policy/policy -> validation -> policy/policy"