[MM-69229] Remove orphaned PermalinkPreviews feature flag (#37147)

The PermalinkPreviews feature flag had no consumers and no SetDefaults
entry (zero-value false). The feature is gated by the separate
ServiceSettings.EnablePermalinkPreviews config setting (defaults true).

e2e-tests/playwright/lib/src/server/default_config.ts

@@ -761,7 +761,6 @@ const defaultServerConfig: AdminConfig = {
         EnableSharedChannelsMemberSync: false,
         EnableSyncAllUsersForRemoteCluster: false,
         AppsEnabled: false,
-        PermalinkPreviews: false,
         NormalizeLdapDNs: false,
         WysiwygEditor: false,
         OnboardingTourTips: true,

server/public/model/feature_flags.go

@@ -34,8 +34,6 @@ type FeatureFlags struct {
 	// AppsEnabled toggles the Apps framework functionalities both in server and client side
 	AppsEnabled bool
 
-	PermalinkPreviews bool
-
 	NormalizeLdapDNs bool
 
 	// Enable WYSIWYG text editor