Remove some now unused pgtype code

Most of this is in conversion, and I assume it became unused with some
of the v5 changes and refactors to a codec-based approach.

There are likely a few more cleanups to be made, but these ones seemed
easy and safe to start with.

pgtype/pgtype.go

@@ -1140,25 +1140,6 @@ func (m *Map) Scan(oid uint32, formatCode int16, src []byte, dst any) error {
 	return plan.Scan(src, dst)
 }
 
-func scanUnknownType(oid uint32, formatCode int16, buf []byte, dest any) error {
-	switch dest := dest.(type) {
-	case *string:
-		if formatCode == BinaryFormatCode {
-			return fmt.Errorf("unknown oid %d in binary format cannot be scanned into %T", oid, dest)
-		}
-		*dest = string(buf)
-		return nil
-	case *[]byte:
-		*dest = buf
-		return nil
-	default:
-		if nextDst, retry := GetAssignToDstType(dest); retry {
-			return scanUnknownType(oid, formatCode, buf, nextDst)
-		}
-		return fmt.Errorf("unknown oid %d cannot be scanned into %T", oid, dest)
-	}
-}
-
 var ErrScanTargetTypeChanged = errors.New("scan target type changed")
 
 func codecScan(codec Codec, m *Map, oid uint32, format int16, src []byte, dst any) error {