protobuf: remove the check for reserved field numbers.

Field numbers in the reserved range are semantically valid,
although the protocol buffer compiler may reject attempts to
define fields in this range.

Change-Id: Ib939ed5f4fef09b29047b509829b2f01494bfb6a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/461238
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

encoding/protowire/wire.go

@@ -29,12 +29,8 @@ const (
 )
 
 // IsValid reports whether the field number is semantically valid.
-//
-// Note that while numbers within the reserved range are semantically invalid,
-// they are syntactically valid in the wire format.
-// Implementations may treat records with reserved field numbers as unknown.
 func (n Number) IsValid() bool {
-	return MinValidNumber <= n && n < FirstReservedNumber || LastReservedNumber < n && n <= MaxValidNumber
+	return MinValidNumber <= n && n <= MaxValidNumber
 }
 
 // Type represents the wire type.