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

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

server/channels/store/sqlstore/user_store.go

@@ -1746,16 +1746,6 @@ func (us SqlUserStore) InferSystemInstallDate() (int64, error) {
 	return createAt, nil
 }
 
-func (us SqlUserStore) GetFirstSystemAdminID() (string, error) {
-	var id string
-	err := us.GetReplicaX().Get(&id, "SELECT Id FROM Users WHERE Roles LIKE ? ORDER BY CreateAt ASC LIMIT 1", "%system_admin%")
-	if err != nil {
-		return "", errors.Wrap(err, "failed to get first system admin")
-	}
-
-	return id, nil
-}
-
 func (us SqlUserStore) GetUsersBatchForIndexing(startTime int64, startFileID string, limit int) ([]*model.UserForIndexing, error) {
 	users := []*model.User{}
 	usersQuery, args, err := us.usersQuery.