A Holocaust survivor  banned from speaking to a Brooklyn middle school  last month will mark International Holocaust Remembrance Day with what Jewish leaders are calling a hollow visit from Mayor Mamdani – amounting to nothing more than a “photo op.” Sami Steigmann, 86, who spent his childhood years in a labor camp in Ukraine and was subjected to Nazi medical experimentation, is expecting a visit Tuesday from the socialist mayor – who has been dogged by accusations of fueling antisemitism with