[FAB-11893] Code hygiene: remove unused const

Part of the restructuring of the validator lead to the removal of the identity
deduplication code. A constant used by that code is no longer referenced
and can be removed.

Change-Id: Id0b6fa70f2974502aeced54170a483978bb0f0d5
Signed-off-by: Alessandro Sorniotti <ale.linux@sopit.net>
Signed-off-by: Matthias Neugschwandtner <eug@zurich.ibm.com>

core/handlers/validation/builtin/v13/validation_logic.go

@@ -25,10 +25,6 @@ import (
 
 var logger = flogging.MustGetLogger("vscc")
 
-const (
- DUPLICATED_IDENTITY_ERROR = "Endorsement policy evaluation failure might be caused by duplicated identities"
-)
-
 var validCollectionNameRegex = regexp.MustCompile(ccmetadata.AllowedCharsCollectionName)
 
 //go:generate mockery -dir ../../api/capabilities/ -name Capabilities -case underscore -output mocks/