Remove some dead code (#27196)

Some code is not in use any more, remove them.

modules/setting/server.go

@@ -81,7 +81,6 @@ var (
 	StaticCacheTime            time.Duration
 	EnableGzip                 bool
 	LandingPageURL             LandingPage
-	LandingPageCustom          string
 	UnixSocketPermission       uint32
 	EnablePprof                bool
 	PprofDataPath              string
@@ -103,7 +102,6 @@ var (
 	StaticURLPrefix            string
 	AbsoluteAssetURL           string
 
-	HasRobotsTxt bool
 	ManifestData string
 )