[MM-53086] Remove SendWelcomePost A/B test and feature (#23733)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/timerlayer/timerlayer.go

@@ -10272,22 +10272,6 @@ func (s *TimerLayerUserStore) GetEtagForProfilesNotInTeam(teamID string) string
 	return result
 }
 
-func (s *TimerLayerUserStore) GetFirstSystemAdminID() (string, error) {
-	start := time.Now()
-
-	result, err := s.UserStore.GetFirstSystemAdminID()
-
-	elapsed := float64(time.Since(start)) / float64(time.Second)
-	if s.Root.Metrics != nil {
-		success := "false"
-		if err == nil {
-			success = "true"
-		}
-		s.Root.Metrics.ObserveStoreMethodDuration("UserStore.GetFirstSystemAdminID", success, elapsed)
-	}
-	return result, err
-}
-
 func (s *TimerLayerUserStore) GetForLogin(loginID string, allowSignInWithUsername bool, allowSignInWithEmail bool) (*model.User, error) {
 	start := time.Now()