Remove test rollout feature (#1061)

Required:

Task/Issue URL: https://app.asana.com/0/1205278999335242/1208159133216403/f
iOS PR: duckduckgo/iOS#3536
macOS PR: duckduckgo/macos-browser#3510
What kind of version bump will this require?: Major

Description:

This PR removes the test rollout feature.

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -54,7 +54,6 @@ public enum PrivacyFeature: String {
     case brokenSitePrompt
     case remoteMessaging
     case additionalCampaignPixelParams
-    case backgroundAgentPixelTest
     case newTabPageImprovements
     case syncPromotion
     case autofillSurveys
@@ -169,11 +168,6 @@ public enum DuckPlayerSubfeature: String, PrivacySubfeature {
     case enableDuckPlayer // iOS DuckPlayer rollout feature
 }
 
-public enum BackgroundAgentPixelTestSubfeature: String, PrivacySubfeature {
-    public var parent: PrivacyFeature { .backgroundAgentPixelTest }
-    case pixelTest
-}
-
 public enum PhishingDetectionSubfeature: String, PrivacySubfeature {
     public var parent: PrivacyFeature { .phishingDetection }
     case allowErrorPage