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/validation.cpp

@@ -2201,15 +2201,9 @@ bool CheckInputScripts(const CTransaction& tx, TxValidationState& state,
                 // string by reporting the error from the second check.
                 error = check2.GetScriptError();
             }
+
             // MANDATORY flag failures correspond to
-            // TxValidationResult::TX_CONSENSUS. Because CONSENSUS
-            // failures are the most serious case of validation
-            // failures, we may need to consider using
-            // RECENT_CONSENSUS_CHANGE for any script failure that
-            // could be due to non-upgraded nodes which we may want to
-            // support, to avoid splitting the network (but this
-            // depends on the details of how net_processing handles
-            // such errors).
+            // TxValidationResult::TX_CONSENSUS.
             return state.Invalid(TxValidationResult::TX_CONSENSUS, strprintf("mandatory-script-verify-flag-failed (%s)", ScriptErrorString(error)));
         }
     }