common.groovy: remove commented-out reference_clone line

Reference clones are being removed from 2.5.0 entirely, so this
dead, already-commented-out option in provide_clone() no longer applies.

Jira: CMK-33523
Change-Id: I625cb3ad0522998144366ce9953e614ddb16863f

Author: Synss

buildscripts/scripts/utils/common.groovy

@@ -58,7 +58,6 @@ provide_clone = { repo_name, credentials_id ->
             branches: [new hudson.plugins.git.BranchSpec("FETCH_HEAD")],
             extensions: [
                 [$class: 'CloneOption',
-                 // reference: "${reference_clone}",
                  timeout: 20,
             ]],
         ]);