validation: Remove RECENT_CONSENSUS_CHANGE validation result

The *_RECENT_CONSENSUS_CHANGE variants in the validation result
enumerations were always unused. They seem to have been kept around
speculatively for a soft fork after segwit, however they were never used
for taproot either. This points at them not having a clear purpose.
Based on the original pull requests' comments their usage was never
entirely clear:
https://github.com/bitcoin/bitcoin/pull/11639#issuecomment-370234133
https://github.com/bitcoin/bitcoin/pull/15141#discussion_r271039747

Since they are part of the validation interface and need to exposed by
the kernel library keeping them around may also be confusing to future
users of the library.

src/bitcoin-chainstate.cpp

@@ -253,9 +253,6 @@ int main(int argc, char* argv[])
         case BlockValidationResult::BLOCK_CONSENSUS:
             std::cerr << "invalid by consensus rules (excluding any below reasons)" << std::endl;
             break;
-        case BlockValidationResult::BLOCK_RECENT_CONSENSUS_CHANGE:
-            std::cerr << "Invalid by a change to consensus rules more recent than SegWit." << std::endl;
-            break;
         case BlockValidationResult::BLOCK_CACHED_INVALID:
             std::cerr << "this block was cached as being invalid and we didn't store the reason why" << std::endl;
             break;