Remove checkpoints

The headers presync logic should be enough to prevent memory DoS using
low-work headers. Therefore, we no longer have any use for checkpoints.

src/consensus/validation.h

@@ -63,7 +63,6 @@ enum class BlockValidationResult {
     BLOCK_MISSING_PREV,      //!< We don't have the previous block the checked one is built on
     BLOCK_INVALID_PREV,      //!< A block this one builds on is invalid
     BLOCK_TIME_FUTURE,       //!< block timestamp was > 2 hours in the future (or our clock is bad)
-    BLOCK_CHECKPOINT,        //!< the block failed to meet one of our checkpoints
     BLOCK_HEADER_LOW_WORK    //!< the block header may be on a too-little-work chain
 };